Floatation Mechanisms in Modern and Fossil Cephalopods

Adv. mar. BWZ., Vol. 11, 1973, pp. 197-268

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHALOPODS E. J. DENTON AND J. B. GILPIN-BROWN The Plymouth Laboratory of the Marine Biological Association of the United Kingdom, Plymouth, England

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I. Introduction . .. .. .. Animals Without any Special Buoyancy Mechanism Buoyancy Given by Fats . . .. .. . . Buoyancy Given by Tissue Fluids . . .. .. Buoyancy Given by Gas Spaces . . . . . . VI. Buoyancy in Fossil Cephalopods . . .. .. . A. The Fine Structure of the Siphuncle B. Posture .. .. .. C. Liquid in the Chambers of the Shell . D. Strength of Shell .. .. VII. Conclusion . .. .. . . . . V I I I . Acknowledgements . IX. References .. ..

11. 111. IV. V.

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197 200 201 201 213 266 267 268 261 264 264 264 264

I. INTRODUCTION We can measure the buoyancy of an aquatic animal by weighing it in air and in the sea water in which it lives. If it has zero weight in such water we say it is neutrally buoyant but, in practice, exact zero weight is rarely recorded and an anaesthetized or dead animal tends t o sink or to float. I n the f i s t case, the weight can be measured directly on a balance while, in the second, the weight which must be added to bring the animal to neutral buoyancy can be found. I n the discussion which follows we shall distinguish between these two conditions by referring to the sinking animal as having a positive weight and the floating animal as having a negative weight in sea water. Having determined the direction and extent of their departures from neutral buoyancy we can compare the buoyancy of different animals by expressing their weights in sea water as percentages of their weights in air. Animals differ greatly in buoyancy. A muscular animal, without a special buoyancy mechanism, e.g. the common squid Loligo, may have a weight in sea water of about 4 or 5% of its weight in air, whilst many oceanic squid, e.g. Histioteuthis, are very close indeed t o neutral buoyancy and have weights in sea water of less than 0.1% of their weights in air (Table I). 197

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E. J. DENTON AND J. B. QILPIN-BROWN

Neutral buoyancy could obviously be of great advantage to a pelagic animal for it would not have to work merely to stay at one level in the sea. An active control of buoyancy would be even more useful. Submarines and bathyscaphes are made to have specific gravities very close to that of sea water and so can remain at a chosen depth with little effort and they also use changes in specific gravity to move from one depth to another. In our own experiments, many of which are described below, we sometimes measured densities in g/ml and sometimes specific gravities. Density is defined as mam per unit volume, and specific gravity is here taken as the mass of a body relative to the mass of an equal volume of TABLEI. WEIGHTSIN AIR AND SEAWATEROF OCEANICSQUID Air (9)

Hktioteuthk meleagroteuthia Hktioteuthk sp.

Chiroteuthb veranyi Chiroteuthk sp. Mastigoteuthk sp. Octopoteuthia danae

39.5 25.4 49.0 2.7 0.836 20.0

35.0 3.0

90.0

6.9 17.0 120

A

NUMBER OF

Sea water

AirlSea water

(%I

(mg)

+ 10 + 50 + 57

~

f 2

+

nil

4.9

+ 21 - 2 + 92 + 9.5 + 10 + 520

~

+ 0.025 + 0.2 + 0.116 + 0.074 + 0.59 nil

+ 0.06 - 0.006 + 0.103 + 0-16 + 0.059 + 0.43

water, both volumes being measured at ambient temperature. The differences between the numerical values of densities and specific gravities defined in this way are all trivial in relation to the results discussed below. A useful way of thinking of the buoyancy of an animal is to draw up what might be described as a, buoyancy balance sheet, putting on one side those components which are denser than sea water and which will, therefore, tend to “ sink ” the animal, and on the other side the components which are less dense than sea water and so will tend to float ” it. The sinking components of animals are those whose specific gravities are greater than that of sea water (i.e. for the Atlantic Ocean greater than about 1.028). These components are principally the proteins of their tissues, especially muscles, and their skeletons. The effective densities in solution given for a number of proteins by Hsber (1945) are all close t o 1.33 and skeletons often contain calcium I‘

199

FLOATATION MECHANISMS I N MODERN AND FOSSIL CEPHALOPODS

salts whose density is about 3, and sometimes chitin which is also denser than sea water. The relative proportions of different sinking components will vary from animal to animal. Thus in a very muscular animal, like Loligo, the most important sinking component is the protein of its muscles, while in the pearly Nautilus, the dense minerals of its shell have a much greater weight in sea water than. the proteins of its tissues. On the other side of the balance sheet the principal floating components of animals are those whose densities are less than that of sea water and these can be fats, certain body fluids and chambers filled with gases. All animals have some fat and, since its specific gravity is generally close t o 0.9, it will always provide some lift. Sometimes this lift is quantitatively unimportant, sometimes it is the principal buoyant component. I n some fish the fat content is as high as 25% of the total body volume and the lift which it gives is sufficient to make them neutrally buoyant even although they are very muscular (Corner et al., 1969).

If a marine animal’s tissue fluids were replaced by pure water this could give a net lift in the sea of over 2% of the animal’s weight in air but, since the tissues of animals cannot function in pure water, this extreme condition is not found. I n some fish the tissue fluids are, however, markedly hypotonic and so less dense than sea water and the diluteness of the fluids in fish eggs is often very important in making them float (Milroy, 1897 ; Lasker and Theilacker, 1962). Even with tissue fluids isotonic with sea water, an animal can still gain some buoyancy by changing the kind of ions and molecules which its body fluids contain. Gross and Zeuthen (1948) in their work on the diatom Ditylum studied the effect on specific gravity of changing the proportions of the common ions of sea water without change in the total osmotic concentration. They showed that sume gain in buoyancy can be made by such changes and that it is particularly helpful t o exclude the divalent ions, calcium and sulphate. A much greater buoyancy gain can be achieved by replacing the common ions in sea water by other substances. A very high concentration of hydrogen ions in the general body fluids of an animal would help greatly but such a high concentration has not yet been found, nor indeed does it appear very likely. Very high concentrations of ammonium ions, which appear almost equally improbable, are however used. A solution isosmotic with sea water of specific gravity 1.026 but containing ammonium chloride and sodium chloride has a specific gravity of about 1.010. The clearest quantitative example of the use of ammonium ions for buoyancy is found in some oceanic squid (see p. 200).

4

A.P.B.-~~

+

9

200

E. J. DENTON AND J. B. OILPIN-BROWN

At 1 atm pressure air has a density of only about 6 5 that of water and even a t 100 atm the density of a gas like N, is only about that of water. Gas spaces offer, therefore, the most obviously effective way of giving an animal lift in a small volume and buoyant gas spaces either in bladders, like those of seaweeds, fishes, or siphonophores, or in chambered shells, like those of Sepia and Nautilus, have long been recognized. Several general methods of using gas spaces seem to be possible : (1) To have a chamber with compliant walls so that the gas pressure inside the chamber equals that of the external hydrostatic pressure of the sea. Some special mechanisms will then be needed t o secrete gases and to prevent these gases going into solution once secreted; ( 2 ) To have a chamber with strong walls within which a gas space can be created by the active removal of liquid. The gases inside this rigid walled chamber need not give a pressure equal to that of the sea but the walls must be capable of withstanding the difference in pressure across them. If these gases are in diffusion equilibrium with the gases dissolved in the living tissues surrounding the chamber they will only exert a combined pressure of about 1 atm ; this is because the gases dissolved in the sea are a t all depths roughly in equilibrium with the gases in the atmosphere above the sea. If a gas space of this kind is to be used there must be some mechanism capable of pumping liquid out of the chamber against the pressure to which the animal is subjected. This method is used by the cephalopods with chambered shells, Sepia, Nautilus and Spirula; (3) An animal might fill a rigid walled box with air a t the surface, seal it completely so that water cannot enter and then use it down to the depth at which the box would implode; (4)An animal might pump gas into a rigid chamber a t one depth and so expel the liquid which it contains, seal the chamber to water, and then use it to give buoyancy at much greater depths. The pressure of gas inside the box would not then need to match the whole external hydrostatic pressure of the sea and the box could be less strong. Suppose, for example, the gas pressure inside the box were 5 0 atm and the external hydrostatic pressure 7 5 atm, the box would then have to withstand only the difference in pressure of 25 atm. Some such system was suggested by Bruun (1943) as a possible mechanism for the shell of Spirula but later work shows that it is not used by this animal.

+

11. ANIMALSWITHOUTANYSPECIALBUOYANCY MECHANISM Many modern cephalopods have no obvious buoyancy mechanism. Some, like the common octopus Octopus vulgaris Lamarck, would get little advantage from being neutrally buoyant, yet there are many

FLO.,ITATION MECHANISMS I N MODERN AND FOSSIL CEPHALOPODS

201

others which are pelagic and which might be expected to derive great benefit from neutral buoyancy but which are, nevertheless, appreciably denser than sea water. Thus, Loligo forbesi Steenstrup, the common coastal squid, is about 4% denser than sea water and, like the mackerel, must swim all its life if it is to avoid sinking. The detailed buoyancy balance sheet of Loligo is not known but its principal sinking component is certainly muscle. Its shell is reduced to the very thin, transparent " pen " which has a weight of only 0.6% of that of the whole animal and which, since its specific gravity is only about 1.2, weighs very little in sea water (Denton and Gilpin-Brown, unpublished observation). Similar figures are found for common oceanic squid such as Ommastrephes and Todarodes and, like some very active fish, e.g. tunny fish, these animals presumably do swim all their lives and obtain the lift needed to stay in mid-water by swimming. 111. BUOYANCY GIVENBY FATS All cephalopods will derive some lift from the fat they contain but sometimes this lift is of trivial importance. For example, in a specimen of Histioteuthis reversa (Verrill), recently examined aboard R.R.S. Discovery, the viscera (which consisted largely of the liver) were slightly buoyant with a weight in sea water of about - 0.4% of their weight in air (Denton and Gilpin-Brown, unpublished observation), but the viscera only provided a small fraction of the lift required to produce neutral buoyancy in the whole animal and, as we shall see below, Histioteuthis achieves neutral buoyancy by other means. Dr C. F . E. Roper reports (personal communication) that some oceanic squid in the Onychoteuthidae, the Gonatidae and the Ommastrephidae have exceptionally large livers containing great quantities of oil. Thus Illex illecebrosus (Lesueur) has a huge liver completely permeated with oil which flows out when the liver is cut. A special study of the buoyancy of these squid would be interesting for it may well be that they resemble the neutrally buoyant deep-sea squaloid fish which have very large livers containing a great amount of low density oil and which must have a very special control of their fat metabolism (Corner et uZ., 1969). The squid Bathyteuthis may well have these properties for Roper reports that it swims very slowly and often hangs nearly motionless in the water. Its liver is large for the size of the animal and consists of two parts, the anterior part containing two large chambers filled with a reddish-orange oil. GIVENBY TISSUEFLUIDS IV. BUOYANCY As we have seen, some lift can be obtained from tissue fluids which are hyposmotic to sea water. The cephalopods, as a group, have, how-

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E. J. DENTON AND J. B. OILPIN-BROWN

ever, tissue fluids rather close in osmolarity t o the sea water in which they live. The only marked exception to this rule is the liquid which is found within the chambers of the chambered shelled cephalopods, Sepia, Nautilus and Spirula, and this liquid in itself plays only a minor role in providing lift. Animals which have very small amounts of sinking material, for instance the gelatinous medusae, ctenophores, tunicates and molluscs, can achieve neutral buoyancy by having body fluids which are isosmotic with sea water but in which sulphate has, to some degree, been replaced by chloride (Denton and Shaw, 1961). Most cephalopods have far too large a proportion of protein for this mechanism to be effective but the exclusion of sulphate has an important role t o play in the buoyancy of the gelatinous octopod Japetella diaphana (Hoyle). One of these animals was recently examined aboard R.R.S. Discovery (Denton and GilpinBrown, unpublished observation). This animal, which weighed 88 g in air, was found t o weigh only 0.12 g in sea water. By cutting the animal into different parts and observing these separately in sea water, it was found that only the arms and mantle were initially positively buoyant and this buoyancy did not last long for they soon sank. Since it appeared that their floating component was leached out of the tissues when the cut tissues were in sea water, a fresh piece of mantle was squashed and the expressed liquid analysed. Although the liquid was isosmotic with Eiea water, its sulphate concentration was only about half that of sea water. Calculation showed that this reduction in the sulphate concentration would provide sufficient lift to balance about half of the weight of the sinking components of this animal and that, although the lift given t o Japetella by its body fluids was small, it was sufficient t o bring this very watery animal about half way towards neutral buoyancy. Unlike some other squids the body fluids of this animal contain only trivial amounts of ammonium. It is not known how widespread this method of reducing the animal’s weight in sea water is in the octopods but many deep water species have a similar. gelatinous consistency and Roper and Brundage (1972) suggest that the gelatinous cirrate octopods, so beautifully illustrated in their paper, may be neutrally buoyant, allowing them to “ float ” above the bottom. The replacement of other cations by ammonium is certainly the buoyancy mechanism in the Cranchidae, a family of pelagic oceanic squid whose anatomy, physiology and behaviour are all determined by their possession of a large liquid-filled buoyancy chamber. The buoyancy of five species of the Cranchidae has so far been investigated. These are Verrilliteuthis hyperborea Steenstrup, Galiteuthis arrnata Joubin, Helicocranchia pfefj’eri Massy, Taonius megaEops (Prosch) and

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHALOPODS

FIQ.1. Photograph of living Helicocranchia Rfefleri (Magnification

203

X 2). The animal is not actively swimming and so it hangs in the sea water with its head downwards.

204

E. J. DENTON AND J . B . GILPIN-BROWN

Cranchia scabra Leech, and all have essentially the same buoyancy characteristics. Fresh undamaged specimens, in sea water, hang almost motionless without effort, rising or sinking only very slowly even when they are not swimming (Fig. 1). All are clearly very close to being neutrally buoyant. It is known from the work of Chun (1910), who described the anatomy of Cranchia scabra in detail, that the coelom is exceptionally large in this family. We have recently studied the anatomy of Helicocranchia (Denton et al., 1969) which is very like Cranchia. I n both species the coelom, whose wall is extremely thin and diaphanous, is enormous and occupies almost all of the mantle cavity, extending anteriorly a t least as far as the visceral ganglion and statocysts and posteriorly to the end of the mantle cavity (Fig. 2). It consists of one individual compartment. The renal sacs which join in the midline are quite distinct from the coelom but lie within it just behind the liver. They have very long lateral lobes which extend along the afferent branchial vessels to include the gill hearts. Each renal sac communicates with the coelom only through a long and narrow reno-pericardial canal and with the exterior through a small renal papilla. Investigations have shown that the lift necessary to balance the denser tissues of these squid arises from the fluid contained within this large coelom. Denton et al., (1969) found the weights in air of four specimens of Helicocranchia and one of Verrilliteuthis, punctured their coeloms and drained off and measured the volumes of the coelomic fluids. The animals without their coelomic fluids sank quickly and could easily be weighed under sea water. The details of these measurements and calculations based on them are given in Table I1 which shows that the coelomic fluids accounted for almost two-thirds of the weights of these animals in air. The specific gravity of the animals without their coelomic fluids was about 1.046, i.e. greater than that of sea water, whilst the coelomic fluids had specific gravities of about 1.010, i.e. closer to that of distilled water than to that of sea water. The low specific gravit,ies found for these fluids could not be given simply by replacing the " heavier " ions of sea water by the " lighter " ones, e.g. replacing calcium and magnesium by sodium, and sulphate by chloride. Determinations of freezing point showed that the coelomic fluids were almost isotonic with sea water and that their low specific gravities could certainly not be explained by their having osmolarities of about 40% of that of sea water. The low specific gravities are in fact given by replacement of almost all the cations of sea water by ammonium. A buoyancy mechanism such as this raises interesting

FLOATATION MECHANISMS IN MODERN A N D FOSSIL CEPHALOPODS

205

FIG.2. Cranchia scabm. The mantle of the squid has been cut along the mid-ventral line and the large liquid-filled coelomic cavity can be seen. The head of the animal is upwards. M, mantle; C.C., coelomic cavity; Si, siphon. (After Chun, 1910). Magnification X 3.

206

E. J. DENTON AND J. B. OILPIN-BROWN

TABLE11. CRANCHIDSQUID Verrilliteuthis

Helicocranchia Specimen-reference number Weight of animal in air (g) Weight of animal minus coelomic fluid in air (g) Weight of animal minus coelomic fluid in sea water (g) Density of animal minus coelomic fluid 20°C Volume of ceolomic fluid (ml) Weight of coelomic fluid as yo total weight in air

3.02

23.01

9.01 9

3.0

5.0

3.5

4

0.046

-

0.074

0.096

0.22

1.041 5.5

10

1.047

1.050 -

1.045 11-5

-

52

15

8.5

65

28.02 15

5.5

67

61

16.02 22

10.5

~~~

Dashes indicate no data.

physiological problems for these exceptionally high ammonium concentrations have t o be retained, secreted and stored. It will be observed (Table 111) that all the coelomic fluids were very acid with an average pH of around 5 . This may well account for the retention of ammonia within the coelom. Small unionized molecules often penetrate biological membranes rather readily as compared with ionized salts (Krogh, 1939). Now consider the simple situation in which a strongly ammoniacal fluid in one compartment (e.g. the coelom TABLE111. PROPERTIES OF COELOMIO FLUIDS OF CRANCHIDSQUID Helicocranchia Specimen-ref. no. 9.01 Density (room 1.010 temperature) Freezing point 1.7 depression ("C) Volatile base mequiv/l Ammonium (mm) 475 Na+ (mM) 80 K+ (miv) Cl- (rnM) 657 4.9 PH ~~~

Dashes indicate no data.

28.02 1.010 1.8

Galiteuthis 8.03 1.012 1.6

29.01 1.011

16.02 1.011

1.8

1.8

466

503 480 83 3.2 637 5.6

470 85

395 150

470 89

642 5.2

589 4.7

555 5.8

3.5

-

Verrilliteuthis

-

sea

wate? -

1.026 1-9

49 1

-

568

-

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHALOPODS

207

in the cranchid squid) is separated from another fluid in a second compartment (e.g. the blood) by a wall impermeable t o NH,+ but through which unionized ammonia can pass. I n both compartments the reactions NH,+ OH- + NH,OH (1)

+

+

NH,+ +NH, H+ (2) will rapidly approach an equilibrium in which relatively very little ammonia is present as NH,OH and where for reaction (2)

The diffusion of unionized ammonia across the separating wall w-ill tend t o make its concentration the same in both the coelom and the second compartment. At equilibrium [NH,

[NH,+] (coelom)

+I (second compartment)

-

[H+](coelom) [H +](second compartment) = K d 4 )

where K, is the factor by which the ammonium and hydrogen are concentrated Denton (1971). Now the ammoniacal liquids of these animals have a p H around 5 and contain approximately 500 mmol/l. ammonia (including both ionized and unionized forms). The ammonium concentrations found in the bloods of such animals were ones of a few millimoles per litre and the pH values of the blood of squid are reported as being between 7.0 and 7.9 (Nicol, 1960; Potts, 1965). It appears, therefore, that equation (4) could fit reasonably well. The above argument does not, of course, tell us how the high concentration of ammonium is achieved. One possibility, suggested by Jacobs (1940) to account for the accumulation of ammonium in animals, is that an acid fluid is actively secreted into a space and that this traps unionized ammonia molecules diffusing in from the blood stream converting them into relatively impermeant ammonium ions. This would not be a surprising mechanism since, for example, the mammalian stomach and kidney can secrete solutions which are more acidic than the coelomic fluids of the cranchid squid. Potts and Parry (1964) have suggested a similar mechanism t o explain the concentration of ammonium in the renal sacs of S e p i a and this fits most of Potts’ observations on ammonia excretion of Octopus d o j e i n i (Wiilker). I n 0. dojleini Potts found that much of the ammonia arises in the kidney, perhaps by the deamination of glutamine. Another possibility t o explain the results on these ammoniacal squid is, of

208

E. J. DENTON AND J. B. OILPIN-BROWN

FIG.3. Histioteuthis, about twice natural size.

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHALOPODS

209

course, that ammonium chloride is secreted and that the acidity arises secondarily. I n Sepia, Loligo and Octopus ammonia is secreted in fair concentrations into the renal sacs, presumably by the very conspicuous kidney tissue which envelops the large veins, particularly the afferent branchials. It is then probably excreted partly through the renal pore and partly through the gills. Potts (1965) has shown that in Octopus dojleini the total concentration of ammonia in the blood of the afferent branchial vessels usually increases as it passes through the kidney tissue and that there is subsequently a significant loss of ammonium through the gills. I n Helicocranchia and Cranchia where there is a need to conserve ammonia, a different arrangement exists. The afferent vessels to the gills carry no obvious kidney tissue (Denton et al., 1969) and it seems unlikely that ammonia found in the coelom could have been first secreted into the renal sacs and then passed into the coelom through the very narrow reno-pericardial canal. It seems more likely that in the Cranchidae ammonia is secreted directly into the coelom by some of the structures which lie within it. The absence of much kidney tissue on the afferent branchials would indicate the necessity of reducing the loss of ammonia through the gills for, as we show below, in order to attain neutral buoyancy these squid must retain a very large fraction of their total life’s output of ammonia. The difference in specific gravity between these coelomic fluids and sea water is such that an enormous quantity of fluid is required to buoy up the denser tissues of these animals. Table I1 shows that the actual volume is about twice the volume occupied by the animal’s other tissues. In this respect these cranchid squid resemble the bathyscaphe in which the volume of the observer’s steel sphere is far less than that of its large floatation chamber. Like the bathyscaphe too, the possession of this vast buoyancy tank must inevitably reduce the mobility of these squid. Indeed the coelom is so large and takes up so much of the space within the mantle cavity that the cranchids have of necessity a very specialized respiratory and locomotory apparatus (Chun, 1910; Clarke, 1962). Like other cephalopods, e.g. Sepia (Robertson, 1953) and Octopus (Delaunay, 1931)) the cranchids almost certainly have ammonia as an end product of their nitrogen metabolism. The metabolic economy of this particular buoyancy mechanism is obvious for they merely have to retain a fraction of their normal nitrogenous secretory product to gain lift. This fraction must usually be large. For example, if we assume that the only “ sinking ” component in a cranchid squid is protein of specific gravity 1.33 and the only “ floating ” component is the ammon-

210

E. J. DENTON AND J . B. GILPIN-BROWN

iacal coelomic fluid, then it is easy to show that to attain neutral buoyancy the nitrogen stored as ammonia in the coelom must equal about two-thirds of the total protein and amino-acid nitrogen of the animal. If a cranchid squid was still actively growing and had had throughout its life, a gross growth efficiency of 33% (a value found by Corner et al., (1967) for Calanus) then the animal would have had t o retain in its coelom about 40% of all the ammonia it had produced in the whole of its life (in this calculation we have assumed that ammonia was the only end product of protein metabolism). A further advanta.ge of this mechanism is that a liquid-filled buoyancy chamber has a compressibility close t o that of sea water and so, in contrast t o the gas-filled swimbladder of a fish (Fig. 13), it will not be markedly affwted by changes in external hydrostatic pressure. We now know therefore that a special buoyancy mechanism is operative in the Cranchidae. They have filled their enlarged coeloms

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHALOPODS

21 1

FIG.4. I n this figure we compare the structures of an arm of Histioteuthis (a) which is

buoyant, with an arm from Todarodes (b) which is not buoyant. I t will be noted that in Histioteuthis the arm contains a large amount of vacuolar tissue and its buoyancy can be quantitively accounted for if this vacuolar tissue alone contained ammonium at the concentrations found in extruded liquid. Photomicrographs of transverse sections. Magnifications (a) x 15, (b) x 8.

212

E. J. DENTON AND J . B . QILPIN-BROWN

with aqueous solutions containing principally ammonium chloride a t concentrations almost isosmotic with sea water. I n doing so their coeloms have become so large that locomotion has been reduced, a special respiratory mechanism has been introduced and the squid themselves frequently have a characteristic balloon-like appearance (Fig. 1). The accumulation of ammonium chloride solution for buoyancy is certainly not confined t o the cranchid squid. Clarke et al. (1969) have found that a similar mechanism is present in squid of the families Histioteuthidae, Octopoteuthidae and Chiroteuthidae. I n these three families the buoyant ammonium chloride solution is not, as in the Cranchidae, confined within a single large buoyancy chamber but is distributed throughout many of the tissues of the animal. This means that the shape of these squid is not determined by a single large buoyancy chamber and we find a wide variety of forms. I n these squid some regions of the body, particularly the mantle and arms, were found t o be very buoyant and t o contain ammoniacal fluids very similar in composition t o those found in the coeloms of the cranchids. We saw above that we should expect a fairly muscular animal t o be neutrally buoyant if its ammonium nitrogen were about two-thirds of its total protein and amino-acid nitrogen. Clarke et al. (1969) found that this ratio was 0.69 for a specimen of Octopoteuthis danae (Joubin) and 1.3 for a specimen of Histioteuthis reversa (Verrill) (Fig. 3). Clearly animals like Octopoteuthis and Histioteuthis must derive the lift giving them neutral buoyancy almost entirely from the ammonium chloride rich solutions which they contain. If the ammonium in these animals had been spread uniformly through the whole of the body fluids the concentrations of ammonium in these two specimens would have been 360 and 260 mM respectively. These are extremely high values for ammonium ; they are nevertheless lower than those obtained from liquids which were extruded from the buoyant parts (i.e. mantle and arms) of other specimens of the same species. When the buoyant parts of such animals were examined histologically they were found t o contain very large amounts of vacuolar tissue compared with squid which were not buoyant (Fig. 4). Moreover, their buoyancy could be quantitatively accounted for if these vacuolar tissues alone contained ammonium a t the concentrations found in the extruded liquids and there was very little ammonium elsewhere in their bodies. Recently Denton, Gilpin-Brown and Wright (unpublished observations) have found that the ammonium levels in the bloods of some of these animals are only a few millimoles/litre. The ammonium chloride rich solutions, although not confined within one compartment like the coelom of the cranchid, are proba.bly, therefore, only found in special tissues.

FLOATATION MECHANISMS M MODERN AND FOSSIL CEPHALOPODS

213

The distribution of ammonium within the animal varies greatly from one species to another and sometimes changes markedly even during the life of an animal. I n Histioteuthis the ammonium is distributed throughout the body in such a way that very small forces enable it t o maintain any attitude. I n Mastigoteuthis the arms are very buoyant and this animal lies vertically in the sea with the arms held upwards while in Chiroteuthis most of the buoyancy is contained in the enlarged fourth arms so that in its resting position the buoyancy of these arms brings the animal to about the position shown in Fig. 5a (W. G. Pearcy, 1968, personal communication). The doratopsis larva of Chiroteuthis on the other hand differs greatly from the adult for its buoyant ammonium chloride solution is largely contained in a balloonlike neck which is absent in the adult (Fig. 515). The head, arms and the body of the larva are all denser than sea water and in this stage the centres of buoyancy and gravity must be fairly close together so that, unlike the adult, the larva can easily adopt any posture. A buoyancy mechanism using ammonium chloride is therefore common in the pelagic cephalopods. It is almost certainly used by several other families in addition to the Cranchidae, Histioteuthidae, Octopoteuthidae and Chiroteuthidae. Thus Dr M. R. Clarke tells us that he found that the posterior end of the mantle of a specimen of Ancisterocheirus lesueuri (Anoploteuthidae), recovered from the stomach of a sperm whale, gave off large quantities of ammonia vapour when treated with sodium hydroxide. Very recently (March 1973), together with Dr Clarke, we have shown, on pieces of mantle and arm, that the legendary giant squid Architeuthis also contains very large amounts of ammonium. These examples show that tissue fluids, modified in various ways, play an important role in determining the buoyancy of many cephalopods. Indeed the use of buoyant tissue fluids is probably far more widespread than we yet realise for we know very little about many of the pelagic squid. We do know, however, that many deep-sea squid and octopods have very reduced musculature, ink sac and radula, and perhaps in buoyancy they resemble the very watery deep-sea fish studied by Denton and Marshall (1958).

V. BUOYANCY GIVENBY GAS SPACES The buoyancy mechanisms which we have so far discussed have been found in recent cephalopods whose shell is either absent or reduced t o a thin transparent and flexible pen or gladius. But we must not forget that the very evolution of the cephalopods was probably determined by a particular solution t o the problem of buoyancy which

214

E. J. DENTON AND J. B. GILPIN-BROWN

involved the use of a buoyant shell (Donovan, 1964). The remains of the buoyant shells of large numbers of nautiloids, ammonoids and belemnoids can still be found but there are only three types of such shell t o be found amongst living cephalopods. These are the famous pearly Nautilus, the sole modern example of the Nautiloidea, and Sepia and Spirula which are now the closest living relatives of the many families of the Ammonoidea and Belemnoidea. Morphologically these shells appear very different from one another but we shall see

FIG. 5. Diagram illustrating the change in posture between the adult Chiroteuthis (a) and its doratopsis larva (b). The shaded areas indicate the most buoyant parts. The sketch of the adult was drawn from Professor Pearcy’s photograph of a living specimen in an aquarium jar (this may have slightly altered its posture).

there are very important similarities, particularly in fine structure and in the way in which they function. The shell of Nautilus (Fig. 6) i s robust and coiled. It has a large living chamber within which the animal lives and seems, apart from its coiling, t o be very like the nautiloid orthocone (Fig. 33). The shell of Spirula (Fig. 7 ) is also coiled but it is small and fragile and is enclosed within the animal while the shell of Sepia (Figs 8 and 9), though also totally surrounded by the animal’s tissue, is an uncoiled structure. The chambers of the Nautilus shell are large, up t o about 20 ml in volume and they are bounded by strong dividing septa which can be

215

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHALOPODS

1.5 cm apart. The largest chambers in the cuttlebone, on the other hand, have a volume of only about 2 ml and the dividing septa are about 0.07 cm apart. I n the cuttlebone the chambers are themselves subdivided by about six subsidiary partitions parallel with the main septa, which are held apart by very numerous irregular vertical pillars. 6 t h chamber I

4th chamber Porous siphuncular tube (connecting ring)

5)

tube

Living chamber

FIG. 6. Section of a Nautilus macromphalus shell with thirty chambers illustrating its structure and some of the terms used in the text. The shell is orientated in its natural position and typical levels of liquids within the chambers of the shell are shown (white areas). Although there is a little free liquid in chambers such as 4, 5 and 6, numbered from the most recently formed chamber towards the smallest, this liquid is in low lateral pockets to the sides of the chambcrs and so would not be seen in a median section. (About half natural size.)

In Nautilus, Spirula and the fossil orthocone the siphuncle is a long thin tube which runs through all the chambers of the shell, while in Sepia the siphuncle is almost flat. I n studying the physiology of these shells the siphuncle has a special importance for we know, from the work described below, that it is the only part of the shell which is permeable to liquids and that exchanges of substances between the

216

E . J. DENTON AND J. B. OILPIN-BROWN

insides of the chambers and the animal’s tissues can only take place through its walls. The siphuncular epithelia which lie against the siphuncular walls of the shells of Nautilus and Sepia are like each other and of a very unusual type (Figs 8 and 23). The only cephalopod with a chambered shell which is readily accessible in Plymouth is the cuttlefish Sepia oflcinaZis. The &st study

FIG. 7. Spirula s p i r d a . (a) A diagram showing the animal in its natural swimming

position (and approximately its natural size). The shell, which is internal, is shown; as are two small stern fins. (b)The shell. The hatched part (X)represents the region through which liquids must move. The stippled part (L) is the living part of the siphuncle; the continuous lines (P) the pearly parts of the shell which are impermeable to liquids.

of the general physiology of Sepia was undertaken at Arcachon by the distinguished French physiologist Paul Bert as early as 1867. He made only one experiment of importance on the shell, or cuttlebone. He collected small bubbles of gas from within the cuttlebone by gently grinding it under water and showed that this gas was principally nitrogen containing only about 2 or 3% of oxygen. He suggested that the composition of the gas within the cuttlebone might vary with circumstances in the same way as the gas in the swimbladder of a fish.

FLOATATION MECHANISMS

m

MODERN AND FOSSIL CEPHALOPODS

217

Dorsal

Y

(a)

Chambers

10 cm

I

1

Sub lamellae

Pillars

Epithelium

(b) I

1 mm

Larnellae I

Ampullae

Duct

,ement space

FIG. 8. Structure of the cuttlebone and the siphuncular membrane fromSepia oflcinalis. (a)Diagrammatic longitudinal section through a cuttlebone from an adult animal which would have about 100 chambers. The siphuncular surface is marked xy. (See also Fig. 14.) (b) Detailed longitudinal seotion showing the siphuncular surface of a few chambers. (Simplified from a 50p celloidin section of decalcified bone; stain, acid fuchsin.) (c) Camera lucida drawing of a section of the siphuncular epithelium showing a duct joining an ampulla with one of the spaces in the basement connective tissue.

218

E. J. DENTON AND J. B. GILPIN-BROWN

Rather more recently Denton and Gilpin-Brown (1961a) b and c) and Denton et ul. (1961) have studied the buoyancy mechanism of S. oficina.lis a t the Plymouth Laboratory. One of the things they investigated was the relationship between one chamber in the cuttlebone and the others. To do this the lower, most recently formed,

FIQ.9. Sepiu oflcinulis. A living animal seen from the side. The position of the cuttlebone in the animal is shown in Fig. 14. About half natural size.

chambers of the cuttlebone were punctured from the ventral surface and the cuttlebone then placed under sea water containing Sepia ink and exposed to a vacuum. When no more gas came from the hole in the cuttlebone, atmospheric pressure was restored and the inky sea water allowed to fill the space from which the gas had been taken. The results of such an experiment are shown in Fig. 10 where it may be seen that the vacuum only extracted gas from the punctured chambers and that these chambers filled up completely with ink when atmospheric

FIG. 10. Sepia oficinalis (L.). (A) Transverse section of the cuttlebone. (B)Longitudinal section of the cuttlebone, showing the siphuncular wall z-y. z is posterior to y. A number of chambers were punctured from the ventral side of the cuttlebone, and gas was removed through the hole by vacuum. The pressure was then brought back to atmospheric, and inky water filled the chambers from which gas had been removed.

220

E. J. DENTON AND J. B. OILPIN-BROWN

pressure was restored. This showed that the chambers are quite independent of one another and that within any single chamber, despite its vertical pillars and subsidiary partitions (Fig. 8), gases and liquids were free to move. The cuttlebone is, therefore, functionally a system of independent chambers just as are the shells of Nautilus and Spirula. The cuttlebone’s volume is about 9% of the cuttlefish’s total volume and its specific gravity is around 0-6 so that in an animal weighing 1 000 g in air the cuttlebone will give a lift in sea water of about 40 g. This will just about balance the excess weight in sea water of the rest of the animal and so bring the cuttlefish close to neutral buoyancy. The first clue that the buoyancy of the cuttlefish could change and that the cuttlebone was anything but a dead unchanging organ, came from a study of animals which had been kept in aquaria for some time. Such cuttlefish sometimes seem to become very buoyant and find difficulty in staying at the bottom of their tanks. Two groups of animals were taken, one in which the animals appeared to be very buoyant and another in which the cuttlefish could rest on the bottom of the tank without effort. When anaesthetized the former were found to be less dense than sea water, the latter denser. No bubbles of gas were found either in the mantle cavity or in the softer parts of the body and the bodies without the cuttlebones were all of about the same density. On the other hand, the cuttlebones of these two groups differed markedly in density. The cuttlebones from the less dense animals had densities close to 0.5 while those from the dense animals had densities around 0.65. There was no difference between the two groups in the weight of dry matter per unit volume of cuttlebone, which remained always close to 38%, but these groups did differ in the amounts of liquid the cuttlebones contained. Cuttlebones of density 0.7 contained about 30% of liquid whilst cuttlebones of density 0.5 contained about 10% liquid. The differences in density of these cuttlebones was entirely attributable to differences in the percentage of liquid within them. In the cuttlefish the cuttlebone is therefore not an inert buoyant skeleton since the lift given by the cuttlebone can be changed by altering the relative proportions of liquid and gas spaces which it contains and some region of the cuttlebone must be permeable t o liquid. Anatomically the most likely place for liquid exchanges between the cuttlebone and the rest of the animal is the siphuncle and it was shown that when a freshly extracted cuttlebone was placed under reduced pressure a watery liquid flowed from the surface of the siphuncle but from nowhere else. The siphuncular epithelium-a yellowish coloured membrane which overlies this region of the cuttlebone-was examined histologically. A section through such a siphuncle is shown

FLOATATION MECHANISMS IN MODERN AND FOSSLL CEPHALOPODS

221

in Fig. 8 (c). It has a copious blood supply and also numerous ampullae close to the bone which are connected by very small ducts to the veins. This finding, of a special drainage system in the epithelium which overlies the only permeable part of the cuttlebone, gives support to the idea that in the living animal exchanges between the animal and the shell take place through the siphuncle. The cuttlefish is not a very deep living animal-off Plymouth it is most frequently taken between 30-80 m and it is thought to go down occasionally to about 150 m. It will then be commonly exposed to pressures of around 8 atm and occasionally to pressures of 150 atm. Since the cuttlebone is evidently not an impermeable structure there must be some force which can balance the external hydrostatic pressure of the sea and so prevent the cuttlebone filling completely with liquid. If Bert’s suggestion that the cuttlebone is similar to the swimbladder was correct, it would contain gas under a pressure equal to the hydrostatic pressure of the sea a t the depth at which the animal is living. However, when cuttlebones which had been rapidly dissected from animals, anaesthetized after these had been quickly hauled inboard from about 70 m, were placed under water and punctured with a needle, no stream of bubbles emerged from the holes (Denton and GilpinBrown, 1961a). The cuttlebones could not then have contained gas at a pressure sufficient to balance the hydrostatic pressure of 8 atm which would be found at 70 m depth. I n fact instead of bubbles of gas coming from a punctured cuttlebone water rapidly entered it, showing that the pressure of gas which it contained was not more but less than atmospheric. This result might have been explained on the rather unlikely assumption that there existed within the cuttlebone a gas which was very soluble in sea water. This assumption was shown to be false by puncturing a cuttlebone in air in a closed system when, instead of water, an equivalent volume of air entered the bone. There is, therefore, a partial vacuum within the cuttlebone. The average pressure of gas in a cuttlebone can be estimated by weighing it in sea water both before and after puncturing its chambers under sea water for it will take up more water the lower the initial average pressure of gas within its chambers. I n Fig. 11,which gives the results of puncturing expsriments of this kind, we see that all the cuttlebones studied took up amounts of water which brought their final densities close to 0.73 no matter what the densities had been before puncturing. This indicates that in the living animal when a cuttlebone becomes less dense the average pressure of gas within it falls, and that when a cuttlebone becomes more dense the average pressure of gas rises and that the mass of gas per unit volume of bone remains almost constant whatever the

222

E. J. DENTON AND J. B. GILPIN-BROWN

density of the cuttlebone (Fig. 12). The pressure of the gas within the chambers of the cuttlebone varies about 0.8 atm and never becomes greater than atmospheric. Since the gas pressure falls when liquid is expelled from the cuttlebone, there can be no question of the liquid being expelled by the secretion of gas into the cuttlebone. Some other mechanism than gas pressure must, therefore, be present both for creating and maintaining the gas spaces and for moving liquid in and out of the cuttlebone’s chambers. 1

0 7f:

I

I

0 7C

? 065 ) .

c

0 0)

0

n

r c 060

0 55

0 50

I

I

0

I

I

2

I

3

Time(hours)

FIG. 11. Sepia oflcinalis. Changes in density of cuttlebones of various initial densities punctured under sea water. The times of puncture are shown by the arrows. For clarity the curves are arbitrarily displaced along the abscissa. The broken line refers to a cuttlebone which was not punctured.

We have seen above that if a cuttlebone is placed under reduced pressure its liquid can be extracted, and if this is done under liquid paraffin (to prevent evaporation) samples of this liquid can be obtained for analysis. By measuring the freezing points of liquids obtained in this way, Denton et al. (1961) found the liquid to be usually hyposmotic to sea water and they showed it t o be principally a solution of sodium chloride. This observation suggests that simple osmotic forces between this liquid and the blood (the latter is almost isosmotic with sea water

223

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHALOPODS

(Robertson, 1949 ; 1953)) play a role in holding liquid out of the cuttlebone when the animal is at depth. For instance, in animals taken where the depth of water was 70 m it was shown that this osmotic difference 100

I

1

I

I

0.55

3 50

80 u

-5 ?!

9” 60 c 0 .-

e 1;

a.

5 U

& 40

U m

c

E L 0)

(L

20

0

0.65

0.60

Cuttlebone density FIQ. 12. The composition of cuttlebones of different densities. If the gas within the cuttlebone is brought to a pressure of one atmosphere its volume and hence mass remains constant no matter what the density of the bone. The gas is normally a t less than atmospheric pressure and fills both the volume indicated by the stippled area and also the clear space above this area (see also Fig. 11).

could balance a t least 5 atm of the 7 atm difference in pressure between inside and outside the cuttlebone. This conclusion has recently been confirmed (Denton and GilpinBrown cited by Denton, 1971) in experiments in which cuttlefish were kept a t known pressures in a pressure tank. Their cuttlebones were

224

E. J. DENTON AND J. B. OILPIN-BROWN

then cut sagitally (while under liquid paraffin) so that samples of liquid could be obtained directly from within the chambers. When, for example, an animal caught a t about 70 m was placed quickly under a hydrostatic pressure equal t o that found a t this depth and kept at this pressure for a day or so, only very small differences in concentration were found between samples taken from various positions within the bone. The liquid within the chambers was everywhere markedly hyposmotic to the animal’s blood and the difference in concentration between the liquid within the chambers and the blood was approximately that which would, if placed across a suitable semi-permeable membrane, give an osmotic pressure of approximately 7 atm, i.e. that required t o match the pressure of the sea a t 70 m depth. When animals caught a t about 70 m depth were kept in shallow water for about 2 weeks the liquid within the cuttlebone was everywhere close to being isosmotic with sea water and the animal’s blood. Experiments such as these showed that over the range of pressures corresponding t o the depths a t which the animal lives, the osmotic pressure difference between the blood and the liquid immediately inside the cuttlebone was always sufficient t o balance most of the hydrostatic pressure tending to force liquid into the cuttlebone. We should expect that when a cuttlefish changes depth a small exchange of salt and/or water across the siphuncular membrane could re-establish the balance between osmotic and hydrostatic pressures and that the buoyancy of the animal would hardly change. The insensitivity of this buoyancy mechanism to quick changes in pressure is shown in the experiment summarized in Fig. 13 in which the cuttlefish is compared with a fish with a gasfilled swimbladder (Denton et al., 1961). The gas pressure which is very low in a newly formed chamber of a cuttlebone rises t o about 0.8 atm in those which have been formed for some time. Denton and Taylor (1964) analyzed the gas from chambers of the cuttlebone and found that in the older chambers this gas largely consisted of nitrogen (including argon) (97%), a small percentage of oxygen (2%) and a trace of carbon dioxide. The pressures of the individual gases were in accord with the theory that these gases play an unimportant role in the mechanism of the cuttlebone and merely diffuse into spaces which have been created by forces other than gas pressure. Now although an osmotic difference across a semi-permeable membrane could stop water from diffusing into the cuttlebone, it could not balance the actual crushing effect of the sea’s pressure. The cuttlebone with its septa held apart by numerous pillars is obviously a strong structure arranged so as t o withstand compression. Its

1,

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHALOPODS

2.0

225

I

I I \

\

0'

I

1

\

\

I

2

\

I

3

I

4

I

5

I

6

Atmospheres of pressure FIG. 13. The change in volume of gas space with pressure; continuous line is for gas within the cuttlebone in a living cuttlefish, the pecked line is for the gas in the swimbladder of a living pollack. The volumes of the gas spaces at 1 atm pressure and room temperature are taken as unity.

226

E. J. DENTON AND J. B . OILPIN-BROWN

mechanical strength will, however, determine the depth range within which the animal can live. Recent test,s with a pressure tank on intact dead animals show that the cuttlebone implodes a t pressures equivalent to a depth in the sea of about 200 m. This is a depth greater than that a t which cuttlefish are found (Fig. 26). The buoyancy mechanism in Sepia has then the following characteristics. Each chamber of the shell is independent of the other chambers. The total gas space within the cuttlebone is that which brings the whole animal close to neutral buoyancy but the volume of the gas space in the cuttlebone, and hence the buoyancy of the animal, can

4%

FIG. 14. Diagram summarizing our knowledge of the cuttlebone. The cuttlebone shown here has a density of about 0.6. Liquid within the cuttlebone is shown stippled. I t can be seen that the oldest and most posterior chambers are almost full of liquid. If they were filled with gas, this would tend to tip the tail of the animal upwards. The newest ten or so complete chambers, which lie centrally along the length of the animal, are completely filled with gas. These chambers can give buoyancy without disturbing the normal posture of the animal. The hydrostatic pressure (H.P.) of the sea is balanced by a n osmotic pressure (O.P.) between cuttlebone liquid and the blood. I n sea water the cuttlebone gives a net lift of 4% of the animal’s weight in air and thus balances the excess weight of the rest of the animal (see also Fig. 9).

be changed by varying the amount of liquid which tjhe cuttlebone contains. The liquid immediately inside the cuttlebone is less concentrated the greater the external hydrostatic pressure t o which the cuttlefish is exposed. An osmotic difference exists between the liquids on the two sides of the siphuncular membrane which can balance the major part of the hydrostatic pressure difference between the inside and the outside of the cuttlebone. The pressure of gas inside the cuttlebone is independent of the depth a t which the animal lives and is always less than 1 atm. The cuttlebone is strong enough to withstand the hydrostatic crushing pressure of the sea down t o the depths a t which the animal lives.

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHALOPODS

227

The normal distribution of liquid within the cuttlebone is such that the cuttlefish can remain with its body horizontal in the sea. This distribution is shown diagrammatically in Fig. 14 which summarizes our knowledge of the cuttlebone. This use of liquid within the cuttlebone t o regulate posture is very relevant to theories on the possible postures of fossil cephalopods (see Figs 33, 34 and 35). When the cuttlebone is made less dense the change in the distribution of liquid will tend t o tip the tail of the animal upward. Willey (1902) wrote that one of Cuvier’s regrets was not t o have seen the inhabitant of the chambered shell of Nautilus which, from time immemorial, had been an ornament of the conchologist’s cabinet. This regret is one most of us must share for this beautiful “living fossil ” is only found close t o some of the islands in the Pacific. It is commonly caught in traps (Fig. 15) set a t around 100 m and is occasionally eaten by the natives. Undoubtedly the maximum depth t o which Nautilus can go is greater than 100 m; N . pornpilius L. is caught in the Philippines a t 180-240 m (Griffin, 1900) and Bidder (1962) trapped an animal a t 180 m in New Guinea. However, while it is generally thought that Nautilus can live a t depths greater than 240 m, the maximum depth is by no means certain and the Challenger record of 570 m (Hoyle, 1886) and Dean’s (1901) figure of 450-700 m have t o be viewed with some suspicion (Denton and Gilpin-Brown, 1966). It has long been recognized that the shell of Nautilus gives the animal lift and there has been a good deal of speculation about the way in which it might function. Robert Hooke in 1696 (see Derham, 1726) considered that the chambers of the shell were filled alternately with air or water and that the animal had the power of generating air into and expelling air from them. Buckland (1837a and b ) realized that the living siphuncle penetrated to the oldest chambers and thought that this tube was very extensible. He advanced the ingenious hypothesis that, although the chambers of the shell were full of gas and contained no liquid, the animal could, by forcing pericardial fluid into the siphuncle, cause it t o balloon out into each chamber and so increase the specific gravity of the animal and cause it to sink. Willey (1902), who made the most extensive study of Nautilus in the last century, considers that the chambers of the shell are not individually air-tight but that they are made air-tight and water-tight by the animal completely closing the siphuncular entrance t o the chambers. He, like many others, does not seem t o have considered that considerable forces would be needed t o exclude water from the chambers. This difficulty was understood by Pruvot-Fol (1937) who thought that the siphuncle produced gas which only partially pushed liquid from the

FIG. 15. Sketch illustrating living Nautilus. In the background the Philippine fish-trap in which they are taken. (After Dean, 1901.)

FLOATATIOPT MECHANISMS I N MODERN APTD FOSSIL CEPHALOPODS

I

0

I

2

1

4

I

6

I

8

229

I

10

Vol of liquid in chambers x 100 Vol of chambers

FIQ. 16. Results obtainotl on four sprcimoiis of Nnulilua macro~nphalzcssoon after capturc. Tho lower points ( 0 )aro for t,ho shrlls alone, thc variations in density are almost ontirely duo t o tlifforcnccs in tho liquid contents of thc shclls. (A shell with rclativoly morc liquitl has a smaller gas-fillctl space in its chambers a n d so i t is more tlonnc.) Tho uppcr points ( A ) givc tlcrisitios of tho living tissues alonc (animals romovetl from thcir shclls). Thwo tlonsitiox vary a good deal from one animal t o anothor. The crntral point.s ( 0 )givo tlensitirs of wholc animals (living tissues -+- shclls). Thnsc- cltwsit,ics arc all vory closr to t h a t of sea water. This figure shows t h a t by varying the liquid cont,trnt.of t h r shrll Nautilus brings the density of tho whole animal (living tissrws 1 shrll) close t o t,hat, of sea water even when there arc considorablc difforencos betweon animals in tho tlnnsitios (and/or amounts) of living tinsuc. (Aftor Drnton and Gilpiii-Brown. 1966.)

chambers a nd t h a t this gas was then act,ed 011 directly b y th e external pressure in t he same way as gas trapped in a n inverted glass. Recently Bidder (1962) an d Denton an d Gilpin-Brown (1966) have studied the buoyancy of living Nautilus macromphalus Sowerby in th e Loyalty Islands of New Caledonia. Figure 16 shows results obtained on four freshly caught specimens of Nautilus. It will be seen t h a t in life t he complete animals within their shells were all close t o neutral

230

E. J. DENTON AND J. B. QILPIN-BROWN

buoyancy (they have weights in sea water of about 0.27, of their weights in air). The figure also shows that the animals without their shells varied considerably in density but that these differences were compensated in the whole animal by their shells containing differing amounts of liquid. It thus seems fairly certain that Nautilus, like Sepia, can bring its density close to that of sea water by varying the amount of liquid within its shell. About 80% of the gas space within the chambers of the Nautilus shell is devoted to buoying up the dense material of the shell itself; only part of the remaining space is used to buoy up the animal, the rest being filled with liquid (Table IV). The Nautilus shell, even with no liquid in its chambers, is very much denser than the cuttlebone which in the same condition has a density TABLEIV. Nautilus maeromphalus

Ratios of weights Density Gale. I of slw Vol. of liquid* density A in s/w A in s / w ( A S ) in slw minus Shell -__ Vol. of shell o f shell specimenA in air (A S )in air ( A -1S)in air density density minus (%) of shell liquid+ (%) (%) (%I

+

C E D B

2.6 3.1 3.3 4.4

1.6 2.0 2.2 3.1

+

0.3 0.3 0.2 0.2

0.993 0.954 0.967 0.938

0.032 0.071 0.058 0.087

9.3

4.8

5.3 2.0

0.905 0.907 0.913 0.912

* The volume

of liquid is that which is readily extractable with a hypodermic syringe. Abbreviations :A = animal after extraction from its shell ; S = the shell only ; s/w = sea water.

of about 0.38. This great difference between Nautilus and Sepia is not surprising because the shell of Nautilus not only giyes the animal buoyancy but also provides it with a heavy protective armour and it is, as we shall see, a much stronger shell able to withstand three times greater pressures. We have already shown that in Sepia the only permeable part of the cuttlebone is the siphuncle. This is also true for Nautilus for when a shell, from which the living siphuncular strand had been carefully removed, was continuously weighed under sea water, a steady increase in its weight was observed indicating the slow penetration of sea water into the chambers of the shell. However, as Fig. 17 shows, this increase in weight could be completely prevented by sealing the opening of the siphuncular tube. The siphuncular tube of the Nautilus shell (i.e. that non-living part which joins the septa1 necks of the chambers) is a

231

FLOATATION MECHANISMS IN MODERK A N D FOSSIL CEPHALOPODS

very interesting structure. A diagrammatic transverse section through t h e siphuncle of Nautilus is shown in Fig. I 8 and it will be seen t h a t the siphuncular tube consists of three distinct parts ; an inner horny tube, a chalky tube and u delicate outer pellicle (Owen, 1832). In longitudinal section (Fig. 19) it can be seen t h a t t8he horny tube abuts against t h e t o p of the septal neck which is essentially an invagination of t h e impermeable septal wall. The chalky tube, on t h e other hand,

0

I00

200

30C

Time ( r n i n )

PIG. 17. This rcfws t.o t h r sholl of R’nutilirs. Before timo zero tho animal and its siphunclo were romovotl from t,he shell and sufficient lead added t o the shell t o mako it a little tlcnncr than sea water. The increases in weight indicate the entry of sea wator into t.he chambers of t,hr shell. Bot.wcen tho two arrows the opening of the siphunclo was blockrtl with plast,icine. Tho curvo has been corrected to allow for t.he weight of this plasticine.

is continued around t h e outer surface of t h e septa1 neck while the pellicle of t h e siphuncular tube is continuous with that, which covers t’he whole inner surface of the chamber (Fig. 20). Some simple qualibative tests of permeability and wettability were made on these structures in fresh shells which had been sawn open t o expose them. The chalky tube behaved very like blackboard chalk, which it resembles, for when a coloured watery liquid was placed against its lower end the liquid was immediately drawn upwards t o colour t h e whole tube. The chalky and horny tubes together were shown tlo be porous by placing a thin h.>l.ll-ll

Ill

232

E. J. DENTON AND J. B. QILPIN-BROWN Pellicle

FIG.18. Diagram of a transverse section through the siphuncle from the third newest chamber of a Nautilus; the siphuncular tube surrounds the living tissues; it has three distinct layers. Enlarged about 25 times.

\ /

Horny tube

Porous siphuncular tube

Septa1 neck

/

Septum

/

Living chamber

FIG.19. Nautilus. Diagrammatic longitudinal section through the porous siphuncular

tube in the ragion where it joins the septal neck. This tube consists of three concantric structures, an inner horny tube, a chalky tube, and a n outer thin pellicle. The chalky tube is continued over the septal neck, whilst the pellicle covers the septum a3 well as the septal neck. Enlarged about four times.

FLOATATION MECIIANISMS IN MODERN A N D FOSSII. CEPHALOPODS

233

FIG. 50. A photcrgraph of a f i w chambcsrs of a frrsh Nrcutilus shcll taken imrnctliately aftor it hut1 btwi suwii oprn. This shows that. the prllicle and chalky t,ubes a r r moist and givrs oxarnplrs of thc way in which the pelliclc is thrown into folds around the septal neck. The chalky t,iihe is continuous ovrr t h r septal neck arid porous siphuncirlar tube h u t in wm(’ placrs the chalky tubes cannot be srcn because the prllicle is vrry dark. These strircticrrs are shown diagrammatically in Figs 18 and 19.

234

E. J. DENTON AND J. B. QILPIN-BROWN

roll of blotting paper inside the siphuncular tube and then bringing a coloured watery liquid to the outside of the tubes. The coloured liquid passed readily through the walls of these tubes and soon coloured the blotting paper. I n addition, the thin pellicle was found to be wettable, water spread readily over the septum when the pellicle was

FIQ.21. Nautilus pornpilius. An electron micrograph (Gregoire, 1962) showing the unorientated felting of microfibrils of the wettable pellicle, or periostracum, which covers the siphuncle and the convex side of a septum in a Nautilus shell. Magnification x 34 400.

present but it stood in beads (as it does upon a waxed surface) when the pellicle was absent. In recent years the fine structure of the siphuncle of the Nautilus shell has been studied by a number of workers; e.g. Mutvei (1964a) with the light microscope, Gregoire (1962; 1967) using an electron microscope, and Mutvei (1972a) with a scanning electron microscope.

FLOATATION MECHANISMS I N MODERN A N D FOSSIL CEPHALOPODS

2%

These studies have confirmed the porous nature of the siphuncular wall and have elucidated the relationships between the structures of the shell walls and siphuncle. The periostracum is homologous t o the siphuncle’s thin wettable pellicle (Mutvei, 1964a). The latter is a very thin conchiolin membrane consisting of microfibrillar layers in which the

FIG.22. A scanning electron micrograph (Mutvei, 1972a) showing the loosely-packed aragonite crystals of the porous spherulitic-prismatic layer (the chalky tube) around a septa1 neck of a Nautilua shell. Magnification x 2 250.

fibrils form an unorientated felting (Fig. 21) (Gregoire, 1962). Beneath the periostracum of the shell wall there are three aragonitic layers with varying amounts of conchiolin. The outermost of these layers (the spherulitic-prismatic layer) is greatly modified in the siphuncle and forms the chalky tube. I n the siphuncle it mainly consists of slender unorientated aragonite crystals and the porosity of this layer is clearly

236

E. J. DENTON AND J.

B. CILPIN-BROWN

shown in Mutvei’s recent scanning microscope photographs (1972a) (Fig. 22). I n the shell wall the middle, mother-of-pearl, layer is the densest. It is composed of tightly packed layers of crystals, separated by thin conchiolin membranes. I n the porous region of the siphuncle this layer is represented by the horny tube, which consists only of concentric layers of conchiolin (Gregoire, 1967). Mutvei concludes that this tube (his inner conchiolin layer) corresponds to the conchiolin layers of the mother-of-pearl layer but that the conchiolin membranes are considerably thickened. He also shows (1972a) that the membranes are continuous with the membranes of the nacreous layer of the septa1 neck, against which the horny tube abuts. The third layer of the shell wall (the prismatic layer) is absent in the siphuncle. The siphuncular epithelium in Nautilus is very like that of Sepia. Both have a special drainage system from the shell surface to t h e blood vessels. I n Nautilus this epithelium has been described by Haller (1895), Willey (1902) and, more recently, by Denton and Gilpin-Brown (1966) and Bassot and Martoga (1966). I n transverse section (Fig. 23b) its most characteristic feature (Denton and Gilpin-Brown, 1966) is the manner in which its basement membrane is thrown into very many regular folds whose outer ends reach almost to the surface of the epithelium. The tops of these folds remain expanded and form a regular series of longitudinal ducts within the siphuncular epithelium. Between the folds of the basement membrane there are tall cells with elongated nuclei and it can be seen that they appear fibrous at their bases and adjacent to the longitudinal ducts. Recent studies of this epithelium with an electron microscope (E. J . Denton and Jane Whish, and V. C. Barber, unpublished observations) have shown that this fibrous appearance is due to very numerous mitochondria arranged around small ducts draining into the channels (canaliculi) formed by the folds in the basement membrane. The surface of these cells, which in life is applied to the inner wall of the siphuncular tube, has a conspicuous brush border. At intervals the folds of the basement membrane open out so that the longitudinal ducts are in communication with the spaces beneath the epithelium and these spaces in turn are connected with the venous spaces of the haemocoel. Bassot and Martoga (1966) in their study used a wide variety of histological techniques on material freshly fixed in New Caledonia. They describe cells similar to those observed by Denton and Gilpin-Brown and make the interesting observation that they resemble kidney cells in which rapid selective exchanges are taking place. They also distinguished, a t the base of the epithelium, occasional cells with a large vacuole containing a calcareous concretion, and there was some evidence that

Fro. 23. Nautilus. Camera lucida drawings of transverse sections of the siphuncular epithelium from an animal with an unfinished new chamber. In life the brush borders, here upwards, are appIied to the siphuncular tubes (see Fig. 18). Cut at 12p, and stained in Heidenhain’s haematoxylin. (a) Epithelium from newest unfinished chamber. This chamber was full of liquid. The drainage ducts are barely open and the epithelium has some of the characteristics of that which secretes the calcareous septum. (b) Epithelium from chamber 3. This chamber was almost empty of liquid. Longitudinal drainage ducts are now very well marked.

238

E. J. DENTON AND J. B. OILPIN-BROWN

the vacuole was lined with cilia and they concluded that these cells were pressure sensors. The work on Nautilus and Sepia shows that siphuncular epithelia have the features which Keynes (1969) lists as common ones for secreting epithelia, i.e. the cells are closely joined at one surface of the epithelium, there is a great expansion of this surface by numerous microvilli, the membrane of the other surface is folded to form canaliculi and there are numerous mitochondria in the cells. The epithelium from the A'autilus siphuncle is indeed very similar t o that of the rabbit gall bladder illustrated by Tormey and Diamond (1967). . . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . . . . . . . . . . . . . .

. . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . .

. . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FIG. 24. A standing-gradient flow system like that believed t o operate in the gall bladder. The heavy lines show the movement of solute (this is actively pumped) into a canqliculus. Water follows osmotically and the liquid leaving the mouth of the canaliculus can be almost isotonic with the liquid flowing into the cell from its closed (here left) surface. (After Diamond and Bossert, 1967.) The density of dots indicates the concentration of salt.

A very interesting theory with strong supporting evidence on the functioning of such epithelia has been advanced by Diamond and his colleagues (Tormey and Diamond, 1967 ; Diamond and Bossert, 1967). They suppose that salt is pumped from the epithelial cells into the canaliculi making the solution within them hypertonic and water follows the salt osmotically. As the salt solution flows along the canaliculi towards their open ends, osmotic equilibrium takes place progressively so that the liquid emerging from the canaliculi can, depending on the diameter and length of the canaliculi, the water permeability of their walls, and the solute transport rate and diffusion constant, range in concentration from isotonic to several times isotonic (see Fig. 24). Diamond and Bossert say that the extension of this theory to accwnt for the movement of solution across the brush

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHALOPODS

239

border is much more speculative and they doubt whether the microvilli on this surface are sufficiently long t o build up standing gradient conditions similar to those expected in the canaliculi. The problem of transporting solution across the corresponding surface of the siphuncular epithelium, i.e. that with a brush border which lies against the chamber wall, is more difficult than transporting it across the corresponding surface of the gall bladder for it is presumably close t o this surface that the main drop in hydrostatic pressure between siphuncle and chamber takes place. Diamond and Bossert also make the reasonable suggestion that the site of the solute pumps will be indicated by the main concentrations of mitochondria. For the gall bladder’s epithelial cells these are found around the luminal (closed) end of the canaliculi ; for the siphuncular epithelium they are found closer t o the open ends of the folds in the basement membrane. The structure of the Nautilus siphuncular epithelium supports the conclusion reached by experiment that liquid is removed from or added t o the chambers through the siphuncle. On leaving the chamber this liquid presumably passes into the siphuncular vein and then through the posterior pallial vein into the central venous sinus from which the afferent branchial vessels are given off (Willey, 1902). Whatever the pumping mechanism we can say that in the course of “ pumping out ” a new chamber of Nautilus or Spirula the blood returning along the siphuncle t o the main body of the animal must be first hyperosmotic and later hyposmotic t o sea water. The chambers of the Nautilus shell are of large volume and it is easy t o take gas samples from them and measure gas pressures within them. Tables V and VI give results obtained on the gases in some of these chambers. It will be seen that in the older chambers the total gas pressure tends towards 0.9 atm. This agrees with the hypothesis that the gases within the chambers have arrived there by simple diffusion and are in equilibrium with the partial pressures of these gases in the tissues. Some of these large chambers contain considerable amounts of liquid and this can be extracted from single chambers with a hSTpodermic syringe. It may be seen (Table V I I ) that the most newly formed chamber (chamber 1) has always the greatest volume of liquid and that the volume of liquid declines with successive chambers until from about the sixth or seventh newest it becomes very small in quantity. These liquids were always watery and, as in Sepia, all were found to be hyposmotic t o sea water, several of them very markedly so, and for each animal the liquid in the newest chamber was always the most hyposmotic. Analyses showed that sodium and chloride were the

240

E. J. DENTON AND J. B. GIILPIN-BROWN

TABLEV. THEGAS PRESSURE IN DIFFERENT CHAMBERS OF THE Nautilus SHELL( a t m ) Specimen Chamber no.

1 2 3 4 5 6 7 8 9 10

A

B*

G

D*

0.37 0.54

0.49 0.66 0.84

0.79 0.82

-

0.74 0.76 0.78 0.87 0.88 0.89 0.90 0.93 0.92 0.92

0.83 0.85 0.82 0.90 0.91 0.91 0.90 0.86 0.91 0.91

I*

C

0.1 (ca) 0.65 0.76

t

N

0.89 0.94 0.91 0.92 0.92 0.92 0.92 0.86 0.94 -

* Liquid entering on puncture estimated by weighing animal under sea water for B and I a n d liquid paraffin for D. t The first chamber of I was filled with liquid. TABLEVI. Nautilus (SPECIMEN I OF TABLEV ) Origin of gas sample

Total pressure

Dry air Newest complete chamber Fourth complete chamber Fifth complete chamber

1.00 0.37 0.79 0.82

Partial pressures (atm) N,

0,

A

0.78 0.30 0.72 0.74

0.21 0.029 0.038 0.038

0.009 0.0059 0.0097 0.0097

Oxygen/ nitrogen

(%)

27 9.6 5.4 5.2

Argon/ nitrogen

(%I

1.19 1.Q 1.4 1.3

Note. It was assumed that the chambers were saturated with water a t 26"C, and a partial pressure of 0.033 atm was assumed to be due to this water.

principal ions in these liquids so that the hypotonicities found must, therefore, have been largely produced by a reduction in the concentrations of these ions (Fig. 25). I n Nautilus, as in Sepia, the chambers of the shell are only partially filled with liquids of low osmolarity and the gas within them is always at less than atmospheric pressure. Again, the low pressure of the gas within the chambers means that the shell must be strong enough t o withstand the hydrostatic pressure of the sea to the maximum depth a t which Nautilus lives. The mechanical strength of the Nautilus shell has not been determined by one simple test. Pressure tests on

FLOATATION MECHANISMS I N MODERN AND FOSSIL CEPHALOPODS TABLE

Ghrrniber ~~

~

VII. LIQLJID* C O N T E N T OF D I F F E R E S T O F T H E Nautilus S H E L L ( I d )

241

CHAMBERS

110.

~

1 2 3 4 6

6 7 X 9

10 Total liquid* contrtrt Vol. of liquid* vol. of shrll ~

* The vdumc

of liquid is that. which is readily extractable with a hypodorrnic syringe. Last, soptum in format.ion, chambcr complctdy fillctl wit,h liquid. tr = tracc.

t

I

40

I

I

60

I

I

80

I

I IO(

Notand CI- ('10 sea water)

Analyscs made on liquids from the buoyancy chambers of Nautilus. The and chloride ( 0 )concentrations as percentages of the abscissa show sodium (0) concentrations of these ions in sea water. The ordinate is thc osmotic concentration a s a percentage of that of sea water.

FIQ.25.

242

E. J. DENTON AND J. B. OILPIN-BROWN

shells whose siphuncles had been sealed with epoxy resin were made by Denton and Gilpin-Brown (1966). These showed that the main walls of the shell of N . macromphalus withstood pressures corresponding to a depth of about 600 m, i.e. 60 atm. A similar experiment, but on more shells, has been made by Raup and Takahashi (1966). They found that Nautilus imploded at a maximum pressure of 73 atm. Although at first sight the siphuncular tube hardly seems capable of withstanding a pressure of 60-70 atm the chitinous part is very strong and the pressure is applied from inside a hollow tube of narrow diameter. Denton and Gilpin-Brown (1966) measured the breaking strain of the fixed material of the tube when spread out into a sheet. They calculated that the siphuncular tube of Nautilus is certainly strong enough to withstand pressures corresponding to a depth of about 350 m. Collins and Minton (1967) have made better and more direct measurements on a fresh shell of N . macromphalus by applying pressures directly to a siphuncular tube from which the living material had been withdrawn. They found that the tube was permeable to sea water (with a linear relationship between flow rate and pressure) and able to withstand pressures equivalent to at least 480 m depth of sea water. It seems likely, therefore, that in the living animal the siphuncle is approximately as resistant to pressure as the main walls of the shell and that Nautilus could go down to at least 480 m and possibly 600-700 m before its shell would be crushed or its siphuncular tube burst. By analogy with Sepia and Spirula we might expect Nautilus not to go deeper in life than about two-thirds this maximum possible pressure range, i.e. not deeper than about 400 m (Fig. 26). Collins and Minton made several other interesting observations. They analyzed sea water " filtered " through the siphuncular tube under various applied pressures and found that it was always completely unaltered. They concluded that the tube must be a permeable and not a semipermeable membrane to salts and that it has no properties which could account for the hyposmotic solutions found in the chambers of the Nautilus shell (Denton and Gilpin-Brown, 1966). They showed that at 200 m depth the shell of Nautilus would fill completely with liquid in 2 h if it were not for the living siphuncle and they proved that when both chambers and siphuncular tubes were filled with gas, gas as bubbles could not pass through the wall of the tube even under 10 atm pressure difference between tube and chamber. Since Spirula (Fig. 7) is the closest living relative of the ammonoids and belemnoids, great importance was attached to its study in the last century. The shells of Spirula are very commonly found on some beaches but since it lives deep in the ocean the animal itself was

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHALOPODS

243

Pro. 26. Tho solid lines show tho tlopth rang~swithin which Sepia, Nautilus and Spirula mostly live. The stars show tho depths at which the pressure of the sea will be sufficiont,lygroat t o irnplodc*thrir shrlls.

244

E. J. DENTON AND J. B. GILPIN-BROWN

virtually unknown for very many years. The first complete Spirula to be described was one found at Port Nicholson in New Zealand (Gray, 1845) and a few more specimens were obtained by the oceanographic expeditions towards the end of the century. However, although Spirula still remained an exceedingly rare animal, so great was the interest in it that, at the turn of the century, more was known of its anatomy and morphology than that of many more common cephalopods (Bruun, 1943). The living animal was first observed by Schmidt (1922) during the Dana’s Atlantic cruises, when 95 specimens were obtained, all in mid-water. The morphology uf these animals was studied in considerable detail by Kerr (1 931) who also made some very interesting observations on the evolution and function of the shell. The geographical distribution of Spirula and depth range were first studied by Bruun in 1943. Bruun re-considered his conclusions in 1955 and subsequently M. R. Clarke has made a detailed study of its vertical distribution and growth with the aid of modern closing nets (Clarke, 1969 ; 1970). It has been known for very many years that Spirula is almost neutrally buoyant; for a very early report by Clausen (quoted by Gray, 1845) states that Spirula has “ t h e power of ascending and descending at pleasure ” and Schmidt (1922) noted that the animal could sometimes come to a standstill in mid-water. Denton et al. (1967) report that live animals when freshly caught usually show a very slight positive buoyancy floating very gently upwards and we have recently confirmed this conclusion. We have seen above that in Sepia and Nautilus the shell is impermeable to water except for parts of the siphuncular walls. Mutvei (1964a) has shown that the fine structure of the shells of Spirula and Nautilus are essentially similar ; the shell septa and the septal necks being formed of the same four layers. However, in Spirula the septal necks are very much longer, relative to the size of the chambers of the shell. They extend from one chamber to the next so far that for any given chamber the region corresponding to the porous siphuncular tube in Nautilus lies within the septal neck of the preceding chamber (Fig. 27). The permeability of the siphuncle was tested in a dried Spirula shell. The siphuncle was exposed and small drops of an aqueous solution containing a dye were placed in the angle between a septum and the top of the septal neck. The coloured solution was immediately drawn into the siphuncular tube and, provided more was supplied, it could be extracted from the tube several chambers away with blotting paper. This simple experiment shows that in Xpirukz, as in Nautilus, there is a porous pathway in the walls of the siphuncular tube for the movement of liquids.

FLOATATION MECllANIShfS I N MODERN A X D FOSSIL CEPHALOPODS

245

The shell of 8pirula is small and fragile t o handle. The investigation of t h e contents of the chambers is, therefore, rather more difficult than in Nautilus. Denton et al. ( I 967) have, however, made some observations on the pressure of gascs within the chambers of Spirula shells. They transferred freshly dissected shells t o a bath of liquid paraffin which had been dyed red and successive chambers were then carefully

PIG.27. 1)iagrani ( d t r r .%pprlI6f,lX'd9. ant1 Mutvei, 1964a) of a mrtlitur section through t h r siphuncle of the first few chambers of a Spir'irulrc shell cnlargecl about 30 times. Tho siph~incularwall of a n y ono chamber is a composite striicture formed of a n oiit.rr imperrncahln t,tibo (tho srpt,al npck) and an inner porous tube. Aqueous tlyes placocl at, points like .r are drawn into t,ho siphunclc by a chalky layer (stipplrtl). whilst such liquids placocl at y arc not. absorbotl. P.T., porous tube; Si, siphunclc; S.N.. srpt,al nock. The rrlationship of this part, of t,he shell to that of tho whole shrll can hr swn hy comparing it with Fig. 7.

punctured. If the gas within these chambers had been at more than atmospheric pressure, bubbles of gas would have escaped from them. This never happened. Instead, small amounts of the coloured liquid par:Lffin entered each chamber as soon as it was punctured showing t h a t t h e gases within these chambers must have been at less t h a n atmospheric pressure. This is exactly what is found for both Nautilus

246

E. J. DENTON AND J. B. OILPIN-BROWN

and Sepia. I n one shell in which the first 17 chambers were successively punctured, the fraction filled by liquid paraffin fell progressively over the first four chambers and then remained almost constant. From weighings made of shells before and after puncturing under liquid paraffin it was calculated that the average pressure of gas within these chambers before puncturing must have been about 0.72 atm. A very similar distribution of gas pressures was recorded in successive chambers of some Nautilus shells (Table V) and the average pressure of gas is also close to that calculated for the first seventeen chambers of a growing Sepia. We have seen that the amounts of liquid found within the chambers of the shells of Sepia and Nautilus are those which will make these animals approximately neutrally buoyant, and that the distribution of liquid between the various chambers of the Nautilus shell is very different from that of Sepia. Liquid is always found in some of the chambers of a Spirula shell (Denton et al., 1967), the total amount of liquid in the shell being that which brings the whole animal close to neutral buoyancy. I n the shells of young animals the chambers, apart from the one most recently formed, are completely dry. I n the shells of mature animals liquid is found only in the older chambers (Denton and Gilpin-Brown, 1971). These older chambers are often completely filled with liquid and the concentration of salts is then close to that of sea water. When a gas space is present within a chamber the liquid is often markedly hypotonic to sea water. Since the pressure of gas within the Spirula shell is less than 1 atm, if a shell is not to implode within the living animal, it must be strong enough to support the maximum hydrostatic pressure of the sea t o which the animal is subjected. Bruun (1943) determined the strength of a number of shells which had been cast ashore and concluded that fresh adult shells could probably support external pressures of between 50 and 75 atm corresponding to depths of 500 and 750 m. On similar shells Raup and Takahashi (1966) gave an implosion pressure of 138 atm and Denton et al. (1967) gave 150 atm. Recently Denton and GilpinBrown (1971) were able to test the strength of the shells taken from animals immediately after they had been captured and in anaesthetized and dead animals. They found only a small difference in strength between the shells of very young and of mature animals and all imploded at pressures equivalent to depths of around 1 700 m (170 atm). Clarke (1970) showed that Spirula lives down to 1 200 m. From this review of the main features of the recent physiological work on the shells of Nautilus, Spirula and sepia we see that although

247

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHALOPODS

these shells differ very greatly in gross morphology they have a great deal in common in the way in which they function. They all consist of a series of rigid independent buoyancy chambers which are permeable t o liquids in one region only, the siphuncle. The volume of the gas space within the shell can be changed by altering the amount of liquid it contains and the total gas space within the whole shell is usually approximately that required t o counterbalance the weight in sea water of the rest of the animal. The pressure of gas within these chambers is independent of the external hydrostatic pressure of the sea and is determined by the partial pressures of the gases within the animal’s tissues which, added together, always give a pressure of less than 1 atm. The rigid walls of these buoyancy chambers are strong enough t o withstand the full hydrostatic pressure of the sea down t o the depths a t which the animals are found (Fig. 26). There are, however, a number of outstanding problems which we have not yet considered. The first problem, best studied in Nautilus, is how is the liquid removed from newly formed chambers? When Nautilus is in its natural swimming position, the level of the liquid within a newly formed chamber will be below the porous regions of the siphuncle when the chamber is only half emptied (Fig. 6). Since Nautilus can empty such a chamber before the coiling of the shell during the animal’s growth reverses the orientation of the chamber, we need some explanation of how it can do this. Coupled with this problem is that of how is a new chamber formed? and when is it emptied? A third problem is peculiar t o Nautilus and Spirula for, unlike Sepia, these animals commonly go t o depths well below 240 m, which is the lower limit a t which a simple osmotic pressure between blood and the liquid within the chamber could possibly balance the hydrostatic pressure of the sea (Denton et al., 1961). Three hypotheses seem possible ones to explain how liquid below the level of the permeable part of the siphuncle can still be pumped out of a chamber. The first is that the chambers contain some living cells which transport the liquid t o the permeable regions. I n favour of this hypothesis is the fact that some nautiloids are known t o have laid down very complicated patterns on calcareous deposits within their chambers, and some workers have thought that these patterns could only be laid down by living cells (see p. 259). The second hypothesis is that liquid is transported t o the permeable regions of the siphuncle by evaporation. This would predict that the liquid left behind in a chamber would become progressively more hyperosmotic t o the tissue fluids as more and more liquid was removed and this is not so. The third hypothesis is that the transport of liquid t o the siphuncle depends on A.M.B.-11

11

248

1.J. D1NTON m D J. B. QZWPIN-BROWN

the physical properties of the chamber walls and siphuncle. The thin wettable pellicle lining the inside surfaces of the chamber, aided perhaps by the rocking motion of the animal (Bidder, 1962), would allow liquid to spread and the chalky and horny tubes act as a wick drawing liquid upwards from the lower part of the septal neck and bringing it against the active epithelium (see Figs 19 and 20). This last and simplest explanation seems the most likely one to be true. If so, then as soon as a newly formed chamber of a N a u t i h shell has been about half emptied of its liquid the only link between the main body of liquid in the chamber and the siphuncular epithelium is through the chalky layer of the siphuncle acting as a wick. The main body of liquid is then in effect almost " de-coupled " from the liquid in the chalky and horny layers which are close to the siphuncular epithelium. The advantage of this arrangement is probably that it is only after a relatively long time that the main body of liquid can influence the osmotic relationships between the liquid contained in the calcareous and horny siphuncular tubes and that in the active siphuncular epithelium and its blood vessels. The composition of the main body of liquid will, therefore, have very little influence in deciding whether liquid passes into or out of a chamber. A change in depth will probably involve a change in the equilibrium concentrations of salts on the low pressure side of the siphuncular wall, i.e. in the calcareous and horny tube. The new equilibrium concentrations will, however, only have to be established in the walls of the siphuncular tube and only small amounts of salts or liquids will have to be exchanged across the siphuncular wall to do this. This means that provided the change of depth is not of long duration, very little osmotic work will have to be done to prevent liquid moving either into or out of the chambers. A similar arrangement is found in Spirula for here again when the animal is in its usual swimming position the permeable region of the siphuncle is situated at the highest part of a newly formed chamber. Mutvei (19644 has shown that -a conchiolin layer (or pellicle) overlies the internal surfaces of the septa and the whole length of the septal necks of the Spirula shell ;this pellicle presumably has the same function as that in Nautilus. However, although Spirula usually swims with its head downwards (Schmidt, 1922; Colman, 1954), Dr M. R. Clarke has seen animals swimming head upwards (Denton et al., 1967). Thus in Spirula the main body of liquid within a recently formed chamber may sometimes be brought directly against the permeable region of the siphuncular tube. In Nautilm and Spirula the architecture of the shell is, therefore, such that the main body of liquid within the newly formed chamber is

ELOATATION MEOHANISMS IN MODERN AND FOSSIL C E P E ~ O P O D S

249

effectively r r de-coupled ” from that immediately adjacent to the siphuncular epithelium. Some “ de-coupling ” must also be present in Sepia for the chambers of the cuttlebone become very thin as they approach the siphuncular region and we should expect that slowness of diffusion of salts along the chambers would greatly limit the rate of equilibration between the liquid immediately inside the cutt,lebone and that deeper within the chambers (Denton and Gilpin-Brown, 1961~). Recent work in which the liquid was sampled a t different depths within single chambers confirmed this view. It was shown that when a cuttlefish is placed under an increased pressure it is several - 5 13-

-5

0

n

-

-4

10-

L

e 2u --” L

5

L

t

II

c

,u c

- 3 2

-F

5-

-

9

-

-2

0-

t

I

4

-

days before this change of pressure is reflected by a corresponding change in osmolarity in the liquid deep in the chamber whilst the liquid immediately inside the siphuncle becomes more hypotonic very quickly (Denton, 1971). Clearly a mechanism for pumping liquid in and out of the chamber of a shell might usefully be used t o bring an animal t o some other condition than near neutral buoyancy; it is only in Sepia, however, that short-term buoyancy changes have been observed (Denton and Gilpin-Brown, 196lb). The behaviour of a well-fed cuttlefish can be strikingly affected by light. I n daytime in the laboratory they usually bury themselves in the gravel a t the bottom of their tanks, whilst after twilight they come out of the gravel and swim around until dawn

----I 1 I I 1 I 1 1 I 1 I 1 1 1 I 1 0 41 42 43 FIQ. 29. The effect of light on the buoyancy of Sepia o & i d i s . Changes of weight in sea water of two specimens. The upper curve is for an animal weighing about 330 g and the lower for an animal weighing about 260 g. The ordinate shows the weight in grams of an animal in sea water (a negative weight means that the animal was less dense than sea water). The abcissa shows time in days. The dark areas indicate times of darkness. (After Denton and Gilpin-Brown, 1961b.)

L-1-

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHATAOPODS

261

(Fig. 28). Denton and Gilpin-Brown (196lb) measured the changes in density of such animals and found that when kept in artificial light and dark they showed quick changes in density which could amount to over 1% (Fig. 29). They showed that these changes in density were given by a change in the volume of gas within the cuttlebone. Although these measurements were made under artificial conditions which probably exaggerated the change in buoyancy, there can be little doubt that in the sea similar, though perhaps smaller, diurnal cycles of buoyancy take place and allow the cuttlefish to become denser than the sea water when it lies a t the bottom of the sea in the daytime and to be close to being neutrally buoyant when hunting at night-time. The way in which a new chamber is formed in Spirula has recently been studied by Denton and Gilpin-Brown (1971). Their experiments were performed aboard R.R.S. Discovery which caught a sufficiently large number of animals for examples of various stages in the formation of a new chamber to be seen. We shall give their conclusion in terms of the diagram on Fig. 30. The first step in the formation of a new chamber is the growth of the approximately cylindrical side wall (2). The space within this wall is a t first full of tissue (stage (b)) but this is later withdrawn leaving behind a clear liquid approximately isosmotic t o sea water (stage (c)). The siphuncle must extend in length a t this time. A new septum is now built (stage (d))enclosing the liquid and completing the chamber. The side wall of the shell continues to grow so that the ratios of lengths x : y on Fig. 30 give a measure of the elapsed time since the last septum was laid down. It is not until the side wall becomes relatively long (stage (b)) that the first gas space appears in the chamber as a small bubble, but well before this happens the concentration of salts in the liquid drops t o about one-fifth that of sea water. It seems clear, therefore, that salts are pumped out of the chamber before water. Once the first small bubble has been formed it expands fairly rapidly as the water moves out of the chamber and the salt concentration of the liquid remaining in the chamber rises somewhat. As we have seen already, the gas pressure in a newly formed chamber is very low and only approaches atmospheric pressure after a time equal to that needed to form 4 or 5 more chambers. The new chamber must be structurally complete when the &st small bubble of gas appears for, from that time onwards, it has t o withstand the full crushing pressure of the sea. Some work on Nautilus allows us to add further details t o this story. Denton and Gilpin-Brown (1966) examined a specimen (I)in which the newly formed chamber was still completely filled with liquid and whose septum was only one-third as thick as that of other animals

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E. J. DENTON AND J. B. GILPIN-BROWN

FIG.30. Diagram showing stages in the formation of a new chamber of a S p h l a shell.

(a) The cylindrical side walls of the next chamber to be formed are added as a continuation of the walls of chamber A. At this stage chamber A is still full of liquid isosmotic with sea water (A,about - 1.9'C). (b) A small bubble of gas (under very low pressure) appears in A, but before this happens solutes have been removed from the liquid which it contains and it is now markedly hyposmotic to sea water ( A t about - 0.4'C). (0) More liquid has left A and that remaining is still very hyposmotic to sea water. A clear liquid isosmotic with sea water has been secreted by the tissues (stippled) into the space B; this space is not yet cut off from the animal by a septum. (d) A septum has been built sealing off the chamber B which (like A in (a)) now contains a liquid isosmotic with sea water. The shell is completely embedded in the animal's tissues. The distances z and y referred to in the text are shown. The walls of chambers A and B have been emphrssized. ( A , = Depression of freezing point.)

whose chambers contained gas. This liquid was only very slightly hypotonic to sea water but in composition it differed from sea water, in particular it contained only about one-tenth the concentration of sulphate. This observation supports the hypothesis that as Nautilzls moves forward within its living chamber, it secretes some fluid behind its body. This fluid is then sealed off by a new septum and a new length of siphuncular tube and it is only when the septum and the siphuncular tube are sufficiently strong to withstand the pressure of the sea that the fluid within the chamber is pumped out. Both septum

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253

and siphuncle must be laid down by the tissues underlying them and Denton and Gilpin-Brown noted that, in the specimen they examined, the epithelium lying against the outside wall of the new septum was an active one and quite different from that found in a mature animal in which the shell had been completely formed. The epithelium from the siphuncle running through the liquid-filled chamber (Fig. 23a) was unlike that from a chamber from which liquid had been removed (Fig. 23b) for the longitudinal ducts were not fully opened nor were there as many spaces below the epithelium. It had instead many of the characteristics of the epithelium secreting the septum of the chamber. These histological observations are consistent with the view that at this stage the main task of the siphuncular epithelium is t o secrete the walls of the siphuncular tube itself and not t o pump liquids. The fact that its ducts are not fully opened until the chamber has been structurally completed also fits the hypothesis that they are the route by which liquid is taken from the permeable walls of the siphuncular tube t o the blood system. When the new Nautilus chamber is structurally complete we can assume, from what we already know of Spirula, that a small bubble of gas will be formed. This small bubble of gas will then expand as the liquid is drained from the chamber through the ducts in the siphuncular epithelium. The experiments on the pressures of gas in the chambers of Spirula shells suggested that the liquid is extracted from the newly formed chamber so quickly that it is substantially emptied before much gas can diffuse into it. I n Nautilus too, as Table V shows, the pressure of gas is always lowest in the newest chamber and again the gas pressure rises as we go to older and older chambers until an equilibrium pressure is reached. If the equilibrium between the tissues and the gases in the chambers is attained by simple diffusion then the rate of equilibratioii can be roughly calculated. This has been done by Denton and GilpinBrown (1966) who estimated the area, the thickness, and the diffusion constant for nitrogen for the siphuncle of one of their animals and plotted a curve giving the rise of nitrogen partial pressure towards its equilibrium value of 0.8 atm against time (Fig. 31). When the nitrogen pressures which were found in successive chambers of this Nautilus were marked on the curve, the result suggested that the four chambers studied had been laid down a t approximately 13 days intervals. Measurements on the chambers of a shell from mature specimens of Nautilus showed that the volumes of the gas spaces increase approximately exponentially on going from the smallest t o the largest chamber. If the animal grows a t an exponent,ial rate this would mean that the chambers are laid down at a constant rate and, accepting that the approximate figure of

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E. J. DENTON AND J. B. QILPIN-BROWN

13 days per chamber applies to all the chambers of a Nautilus shell, we find that the length of time it takes a Nautilus to complete its growth after hatching is about one year. This is a very rough estimate, of course, but it does suggest a rapid rate of growth which may also apply to fossil cephalopods. In this connexion we must note a fascinating observation of Schindewolf’s (1968) who found ammonites with the tube-worm Xerpula attached to the ventral side of their shells, both of which must have grown simultaneously. Knowing the growth rate of modern serpulids he calculated that the time for the formation of a new chamber was between a week and a month. Westermann (1971),

0

20

40

60

80

Days FIQ. 31. Nautilua. The curve, which is computed for animal I (see Tables V and VII) from the volumes of chambers and the dimensions of the porous siphuncular tube, shows the rise of nitrogen partial pressure towards 100% of the equilibrium value. The arrows indicate where the partial pressures of nitrogen found in the s a q e animal for chambers 2 , 3 , 6 and 6 fall on this curve. The value for chamber 4 is interpolated. (The chambers are numbered from the most recently formed chamber to the older ones.)

however, believes that many of the Mesozoic ammonoids grew more slowly than living cephalopods and lived for several years. Although the chambers of the cuttlebone of Xepia are very much smaller and laid down at faster rate than those of Nautilus, new chambers are probably formed in a similar way (Denton and GilpinBrown, 1961~). The most ventral and recently formed chamber is almost invariably incomplete and full of liquid while the next oldest chamber frequently has very little liquid and contains gas at very low pressure. As in the other species studied, the pressure of gas in the chambers gradually increases with age until an equilibrium value is reached at approximately the twelfth chamber.

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255

The formation of a new chamber is then basically the same in all three species. As the animal grows its tissue moves away from the last formed chamber secreting n liquid which is sealed off by a new wall to make a liquid-filled space. When this wall is sufficiently strong t o withstand the external pressure of the sca liquid is pumped out of the chamber through the siphuncle. Most of tho problems posed above have a t least been partially answered, one important one, however, remains unresolved. How can Spirula and Nautilus pump liquids out of the chambers of t,heir shells against such high hydrostatic pressures?

VI. BUOYANCY IN FOSSIL CEPHALOPODS I n a short review such as this one it is impossible t o give a general survey of the enormous literature on fossil cephalopods, and we can only discuss briefly some of the problems on which recent work on the buoyancy of living cephalopods throws light. This work has shown that control of buoyancy is a striking feature of the group and that several different mechanisms are used. R7e know however little about the evolutionary history of cephalopods using ammonium or fat for buoyancy and our discussion will be confined t o the evolution of the cephalopods with chambered shells. It is very probable that the emergence of the cephalopods depended on the evolution of their buoyant shells. Donovan (1964) has written a very lucid and interesting account of cephalopod phylogeny and development, and on Fig. 32 we give his summary of the evolutionary relationships between fossil and modern forms. He thinks that the first step in the evolution of buoyancy might have been that of leaving a liquid-filled, tissue-free space a t the apex of a gastropod-like shell, and believes that such a space might arise, either because the rate of growth of the body failed t o keep pace with that of the shell, or because the visceral hump was partially reabsorbed seasonally, perhaps when food was scarce. He argues that, if in shallow water, the liquid was now absorbed from the apical space, the space would then contain gas under low pressure and the animal’s tissues would bulge into it in order to allow its wall t o withstand the pressure of the sea. A later secretion of a septum having an uncalcified region through which liquid absorption could take place (the primitive siphuncle) would allow the animal t o go more deeply. Such a, simple cephalopod has not yet been identified. The first undoubted fossil species is the nautiloid Plectronoceras from the late Upper Cambrian in China; this shell is quite sophisticated, having a number of chambers, well developed septa, and a siphuncle. The evolutionary scheme shown in Fig. 32 is not universally agreed

-

Carnt

-

P-

Sptrula Sepia Teuthoidea Octopoda Varnpyromorpha

FIU.32. Phylogenetic diagram of the cephalopods. (After Donovan, 1964.) P, Plectronoceratidae; B, Bassleroceratidae; E, Ellesmeroceratida (less Plectronoceratidae). The Ammonoidea (all extinct) are shown by the light stipple. Donovan argues that the animals in the unshaded areas, formerly all included in the Nautiloidea, be sub-divided into several major groups. The Coleoidea (contains all the modern cephalopods except Nautilus) are shown by the heavy stipple. (Note: Two forms of spelling are used for some of the Cephalopod orders; e.g. Orthoceratida (Orthocerida) and Ascoceratide (Ascocerida).)

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among palaeontologists. Some authorities, for example Teichert (1967), have advanced alternatives in which, for example, the Ammonoidea had a different origin.

A. The fine structure of the siphuncle Modern work on the physiology of the living representatives of cephalopods with chambered shells has shown that essentially one buoyancy mechanism is employed. This mechanism demands that part of the siphuncle should be permeable yet mechanically strong. In Nautilus and Spirula this has been achieved by relatively simple modifications to the normal shell structure in which calcification is reduced so that the outer aragonite layer becomes porous and the layer corresponding to the nacreous layer is composed only of strong conchiolin (see p. 234 above). These features are recognisable in fossil shells. Mutvei (1964b) pointed out that the minute structure of the siphuncular tube in three orders of fossil " nautiloids " (Michelinoceratida, Actinoceratida and Endoceratida) agreed with that found in Nautilus and Spirula. He stressed that the " connecting ring " (siphuncular tube) in these forms had a structure that must have been permeable to gas and liquid. Recently (1972a), using the scanning electron microscope, he has extended the list to include the Ellesmeroceratida (Pictetoceras), Tarphyceratida, Barrandeoceratida, Nautilida and Orthoceratida. Some of these forms resembled Nautilus very closely. I n other forms the resemblance is less close and the various layers differed in relative thickness from those of Nautilus. I n Pictetoceras and in the Tarphyceratida the outer layer is thicker ; in the Orthoceratida, Barrandeoceratida, and Nautilidait is thinner ;in the Actinoceratida it is absent (Mutvei, 1972a). Other fossil cephalopods have siphuncles which show very great differences from that of Nadilus. Thus in some orthoconic shell fragments, which may belong to the genus Pseudorthoceras (Orthoceratida), Mutvei (1972b) found that the connecting ring consisted only of an outer conchiolin layer and an inner partially calcified layer both derived from the nacreous layer of the shell wall. The relationship of the connecting ring to the other parts of the shell WM also different but Mutvei wm nevertheless convinced that this connecting ring was also permeable and capable of playing its part in the buoyancy mechanism. He noted however that, without the outer porous layer (the " chalky " tube of Nautilus) the liquid within the chambers would be more effectively " de-coupled " from that in the siphuncle than in Nautilus. In the belemnoid Megateuthis giganta Mutvei (1971) found that in each chamber the siphuncle had two horny layers and sandwiched between them there wa8 a layer

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E. J. DENTON AND J. B . OILPIN-BROWN

of spaced aragonite crystals. This is a structure which would almost certainly have been permeable to liquids and gases. The connecting ring of the ammonoids appears to consist solely of concentric membranes which form a comparatively thick tube (Westermann, 1971). The results of all this recent work show that, while structural differences certainly occurred between the siphuncles of different groups, they were almost certainly always permeable to liquid. This suggests that the role of the siphuncle in the buoyancy mechanism of the cephalopods has remained the same in a great variety of forms and over a long geological period.

B. Posture A good deal of thought has been given to those features of the fossil animals which might throw light on their posture and behaviour in life (Fig. 33). Many of the early nautiloids had straight or only slightly curved shells. If the shell of such an animal contained only gas it seems almost certain that the apex of the shell would have pointed upwards and the living chamber downwards. It is generally agreed that nautiloids with short squat shells (brevicones) and some of those with curved shells (cyrtocones) would have had this posture but that animals with slender straight shells (orthocones) would, like the elongated squid of the present day, have swum with the long axes of their shells horizontal. Some authors have suggested that in life these latter shells contained no gas but, with Flower (1955), we find it hard to conceive of a successful group of animals dragging around a useless chambered part of the shell five to ten times the length of the living chamber. Flower writes “ that the straight cephalopods were successful is attested by the abundance of specimens in the Palaeozoic, the several hundred of species recognized, and the large size attained by some of them. The Ordovician endoceroids attained lengths of 12 ft with only one foot of that length occupied by the living chamber: in other stocks shells of four to eight feet developed, in the Silurian, Devonian and Mississippian.’’ Now if the orthocones were buoyant and held their shells horizontally it seems certain that there must have been some weight in sea water at the apex of the shell to counterbalance the weight in sea water of the main mass of living tissue and the calcareous wall of the living chamber. It seems very likely that in many of these animals this counterbalancing weight was provided by the deposition of calcareous material. In the nautiloid order Endoceratida calcareous deposits are found at the apical end of a large tubular siphuncle, the Michelinoceratida

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHALOPODS

259

have such deposits in the chambers of the shell (cameral deposits), whilst in the Actinoceratida they are found in both the siphuncle and apical chambers (Fig. 33). The origin and significance of these various deposits has been the subject of some controversy. If they are not artefacts, as Mutvei (1964b) believes, and were present in the shells of the living animals then they would clearly have been important in

Rutoceratida

Mlchelinoceratida

Endoceratida

--=-=v Plectronoceratidae FIG. 33. Diagrams of some nautiloids. (The evolutionary relationships of these animals is shown in Fig. 32.) Some shells are shown in section and then the siphuncular and cameral deposits (shown in black) can be seen. The possible appearance of the soft parts is based on our knowledge of living cephalopods. (After Flower, 1966.)

determining the posture of these animals. According to Teichert (1964) some deposits were laid down in closed chambers and Flower (1964) has argued strongly that they could only have been formed by living tissues inside these chambers. He noted, for example, that cameral deposits exhibit specific and often elaborate bilaterally symmetrical surface patterns, that they are concentrated on the animal’s ventral sides, that this pattern in the shell does not depend on the orientation of the shells in the sediments and also that some cameral deposits had

260

E. J. DENTON AND J. B. OILPIN-BROWN

surface markings which could only be caused by blood vessels in cameral tissues. The living cephalopods give us no help in settling the question as to whether the deposits were formed in the shell during life or only later. There are no living tissues within the chambers of modern cephalopod shells but then there are no siphuncular or cameral deposits. If an animal had living tissues in closed chambers, resembling those found in the Nautilus shell, and these tissues laid down calcareous material in a controlled way with the animal’s growth, then they would have to be supplied with nutrients (including 0,)and with substances regulating their activity. These substances could only enter the chambers by diffusing through the permeable parts of the siphuncular tubes (the connecting rings). The measurements on Nautilw indicate that if there had been tissues within its chambers

FIG.34. Diagram of possible relationships of the various parts of a belemnoid. The shell is after Phillips (1866-70) cited by Donovan (1964). The reconstruction of the

soft parts of the animal is based partly on our knowledge of modern cephalopods. In our view the chambered part of the shell would probably have had to be relatively much bigger than this diagram indicates for the animal to have been close to neutral buoyancy.

these could have received some O2by diffusion. The liquids within the chambers of Nautilzcs contain, however, only little in the way of nutrients for they are mainly a simple solution of inorganic salts (principally NaCl) (Fig. 26). From the order Michelinoceratida the Belemnoidea evolved and from the Belemnoidea modern Sepia. With the appearance of the Belemnoidea the buoyant shells were, for the fist time, completely enclosed within the living tissues of the animal and with the growth of the phragmacone (the buoyant chambered part of the shell) there was a steady deposition of a solid “ guard ” around the apex of the phragmacone (Fig. 34). This guard could act as a very effective counterbalancing weight to the animal’s living tissues in its living chamber. One remarkable solution to the problem of achieving horizontal stability is found in the nautiloid Ascoceratida. In these animals not only did the apical chambers moult during development but the

FLOATATION MEOIIILNISYSIN MODERN AND FOSSIL OEPHALOPODS

261

FIo. 36. Qloaaoceraa lindalomi (a cephalopod of the order Amoceratida). (A) is of the juvenile form. (B) shows a later stage in which the earlier formed parts of the shell (those of diagram (A)) have been lost and the shell has a different shape in which the buoyant gee-filled chambers are brought over the main body of the animal’s living tissues when the animal has a horizontal posture. The living tissues are shown by the stippIe, their form is based on our knowledge of living cephalopods. (After Furnish and Glenister, 1964b.) ...............

I

...................... B

FIG.36. (A) Perspective diagram of half of the shell of Aecoceraa (at a stage in the life

cycle corresponding to that shown in Fig. 36B. The most recently formed chambers

are linked to the siphuncle only by thin tubes running along the sides of the shell.

(B) Cross section of (A) showing the thin tubes connecting the chambers proper to the wall of the siphuncle. Any liquid within these chambers would be well “decoupled” from the siphuncular walls. (After Furnish and Glenister, 1964b.)

mature shell developed an inflated form so as to extend over the dorsal part of the living chamber (Figs 35: and 36).

C. Liquid in the chambers of the shell Although the evidence of the fossil record gives very good indications of the importance of calcareous deposits in the regulation of buoyancy and posture, it cannot tell us how much liquid existed within the chambers of the shell of the fossil cephalopods in life or where such liquid lay within the shells. In Sepia, Spirula and Nautilus the amount of liquid within the shell varies from one specimen of a given species t o another to bring the healthy animals close to neutral buoyancy,

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1.J. DENTON AND J. B. QILPIN-BROWN

e.g. Fig. 16. There seems little reason to doubt that this would have been true of the fossil cephalopods and that they too would have used liquid within the shell to allow a continuous regulation of buoyancy whilst new chambers were becoming functional at discrete intervals of time. I n mature specimens of Sepia and Spirula the first formed and smallest chambers, i.e. the ones corresponding to the apical chambers of Nautiloids, are generally rewed with liquid (Fig. 14). If similarly the apical chambers of the orthocone shells of the nautiloids contained liquid, this would have been of help in allowing the animals to maintain a horizontal posture but it could not, however, be as effective as the deposition of calcareous material. We believe that the fact that all the buoyant shells of living cephalopods do contain some liquid within their chambers makes it extremely probable that in life the fossil cephalopods had liquid within their shells too. The amount of liquid found within the chambers of the living animals varies considerably between species and from one specimen of a given species to another (Tables V and VII), but the amount of liquid is almost always such as to bring the whole animal (living tissues shell) close to neutral buoyancy (see e.g. Fig. 16). Trueman (1941), who made a careful and interesting study of the floatation of some fossil cephalopods unfortunately made the assumption that the shells of cephalopods, contained only gas and he was led to assume a density for the living tissue of Nautilus and of fossil cephalopods of 1.13. This density would have meant a weight in sea water for the living tissues of more than 9% of the weights in air. This is not the correct value for Nautilue and it is an extremely unlikely one for the fossil animals. Table VIII shows that the weight in sea water of the living tissues of modern cephalopods with chambered shells are all around 3.7% of their weights in air and this would be a sensible value to assume for fossil animals. If we do make this assumption, a re-interpretation of Trueman’s results would lead to the conclusion that the cephalopods which he studied had, in life, some liquid in the chambers of their shells and Heptonstall (1970) has recently made calculations on the buoyancy of some ammonoids with this possibility in mind. One specimen of an ammonoid Buchiceras bilobatum had been previously studied by A. Seilacher. This animal’s shell was encrusted with oysters and Seilacher had argued that since some of these oysters had grown on the lowest portion of the animal it could not have rested in the sediment whilst the oysters were growing. He gave reasons for supposing that the animal must have been alive at this time. Heptonstall estimates that if the animal had preserved neutral buoyancy during the growth of the oysters the shell must have

+

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHALOPODS

263

contained some liquid which could be progressively pumped from the shell t o compensate for the increasing weight in sea water of the oysters. Using reasonable values for the densities of living tissues and shell material he has also made calculations suggesting that the chambers of the fossil ammonoids probably contained appreciable volumes of liquid in life. He gives values for the percentage volumes of the chambers occupied by liquid ranging from 52% fox Ludirigia baylei down t o 7-20% for Buchiceras bilobatum. He does, however, emphasise that there is some uncertainty in these figures because of possible loss of shell material in diagenesis and that the true values might have been TABLEVIII. Sepia officinalis Nautilus macromphalus Spirula spirula Inlernal aheU E x t e n d shell Inlernal shell Approx. wt in air of mature animal shell

+

Wt of living tissues in sea water BB yoof wt in air Vol. of shell

Vol. of animal

+ shell

500 g

5g

3.35% (244.4%) N = 6

3-81yo (3.04-4.47%) N=6

9-3%

36 % (414-32.6%) N=8

8%

0.97 (0.94-0.99

0.63

%

Average density of shell in life

Vol. of liquid in shell Vol. of spaoe inside chambers Approx. implosion depth

1000 g 3.94% (3&4.4%) N = 4

0.62 (0.57-0.65 yo) N = 17

%

1627%

N=7

yo)

5.4%

(9*3-2.0y0) N = 4 240 m

600 m

Similar to Nauti2ua 1700 m

lower. A similar argument, but acting in the opposite sense, would cast doubt on the correctness of diagrams which are sometimes given for the proportions of the various parts of the shells of belemnoids. Thus on Fig. 34 we show after Phillips (1865-70 see Donovan, 1964) the reconstruction of the shell of a belemnoid. If we assume that the shell was enclosed entirely within the living tissues of the animal and these were like the tissues of Loligo, then, even neglecting the weight in sea water of the shell and the guard and assuming the shell t o be completely filled with gas, we find that for the animal and its shell t o be neutrally buoyant the animal without its shell would have only been able t o have a weight in sea water of less than 1% of its weight in air. It seems

264

E. J. DENTON AND J. B. OILPIN-BROWN

possible that the phragmacone (the chambered part of the shell) had a larger volume relative to size of the animal than is often supposed.

D. Strength of shell Ammonoids very often had shells with very complicated shapes and structures and often the proportions of the shell varied considerably between juvenile and adult forms. Westermann (1971) has reviewed the recent literature on ammonoids and the interpretation of shell structure in relation to its function and the life of the animal. He has, in particular, studied the thicknesses of the walls of the shells and of the siphuncle so as to be able to give some idea of the possible depths at which the animals lived. He usually h d s a good correlation between the thickness of the shell and the strength estimate of the siphuncular tube and the conclusions which he reaches on the depth ranges of various ammonoids agree well with previous estimates based on ecological evidence.

VII. CONCLUSION The work described above has shown that cephalopods are unequalled by any other group in the elegance of the mechanisms which they have evolved to regulate their buoyancy. The main future interest of studies on their buoyancy probably lies partly in those on detailed mechanisms, e.g. that whereby ammonium can be secreted in such high concentration in Histioteuthis or that whereby Spirula can pump liquid against very high hydrostatic pressures, and partly in the application of modern knowledge of living cephalopods to the fossil forms. Encouraging results in both these fields have already been obtained. VIII. ACKNOWLEDGMENTS We are very grateful to Mr G. A. W. Battin and Mr D. Nicholson for a great deal of help in the preparation of the figures for this review. IX. REFERENCES Appellof, A. (1893). Die Schalen von Sepia, Spimcb und Nautilus. K . wenaka VetenakAkad. H a d . 25 (7), 106 pp. Bassot, J. M. and Martoga, M. (1966). Histologie et fonction du siphon chez le Nautile. C. r. hebd. Shnc. Acad. Sci., Park, 263, 980-982. Bert, P. (1867). M8moire sur la physiologie de la Seiche. Mern. Soc. Sci. phys. nat. Bordeaux, 5, 114-138. Bidder, A. M. (1962). Use of the tentacles, swimming and buoyancy control in the pearly NautiEw. Nature, Lond. 196, 451-454. Bruun, A. F. (1943). The biology of Spimcla spirmla (L.). Dana Rep. 4 (24), 1-46.

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Bruun, A. F. (1955). New light on the biology of Spirula, a mesopelagic cephalopod. I n “Essays on the Natural Sciences in honour of Captain Allen Hancock pp. 61-72. University of Southern California Press, Los Angeles. Buckland, W. (1837a). “ Geology and Mineralogy Considered with Reference to Natural Theology ”, Vol. I, 618 pp. William Pickering, London. Buckland, W. (1837b). “ Geology and Mineralogy Considered with Reference to Natural Theology ”, Vol. 11, 129 pp. William Pickering, London. Chun, C. (1910). “ Die Cephalopoden ; Oegopsida Wies. Ergeb. Deufach. Tiefsee-Exped. (“ Valdivia ”), 18, 1-401. Clarke, M. R. (1962). Respiratory and swimming movements in the cephalopod, Cranchia scabra. Nature, Lond. 196, 351. Clarke, M. R. (1969). Cephalopoda collected on the SOND cruise. J . mar. bwl. ASS.U . K . 49, 961-976. Clarke, M. R. (1970). Growth and development of Spirula spirula. J . mar. bwl. ASS.U.K. 50, 53-64. Clarke, M. R., Denton, E. J. and Gilpin-Brown, J. B. (1969). On the buoyancy of squid of the families Histioteuthidae, Octopoteuthidae and Chiroteuthidae. J . Physiol., LO&. 203, 49-5OP. Collins, D. H. and Minton, P. (1967). Siphuncular tube of Nautilua. Nature, Lond. 216 (5118),916-917. Colman, J. S. (1954). The “ Rosaura ” Expedition 1937-1938. Part I. Gear, Narrative and Station List. Bull. Br. MW. nat. Hiet. Ser. 2001. 2, 119-130. Corner, E. D. S., Cowey, C. B. and Marshall, S. M. (1967). On the nutrition and metabolism of zooplankton. V. Feeding efficiency of Calanua Jinmarchicus J . mar. biol. Ass. U.K. 47, 259-270. Corner, E. D. S., Denton, E. J. and Forster, G. R. (1969). On the buoyancy of some deep-sea sharks. Proc. R . SOC.Lond. B, 171, 414-429. Dean, B. (1901). Notes on living Nautilue. Am. Nat. 35, 819-837. Delaunay, H. (1931). L’excretion azotbe des invertbbrbs. Bwl. Rev. 6, 265-301. Denton, E.J. (1971). Examples of the use of active transport of salts and water to give buoyancy in the sea. Phil. Trans. R . Soc., Lond. B, 262, 277-287. Denton, E. J. and Gilpin-Brown, J. B. (1961a). The buoyancy of the cuttlefish Sepia oficinalie (L.). J . mar. biol. Ass. U . K . 41, 319-342. Denton, E. J. and Gilpin-Brown, J. B. (1961b).The effect of light on the buoyancy of the cuttlefish. J . mar. biol. Ass. U.K. 41, 343-350. Denton, E. J. and Gilpin-Brown, J. B. (1961~).The distribution of gaa and liquid within the cuttlebone. J . mar. bwl. Ass. U.K. 41, 365-381. Denton, E. J. and Gilpin-Brown, J. B. (1966). On the buoyancy of the pearly Nautilua. J . mar. b w l . Ass. U . K . 46, 723-759. Denton, E. J. and Gilpin-Brown, J. B. (1971). Further observations on the buoyancy of Spirula. J . mar. biol. Ass. U.K. 51, 363-373. Denton, E. J. and Marshall, N. B. (1958). The buoyancy of bathypelagic fishes without a gas-filled swimbladder. J . mar. bwl. Ass. U . K . 37, 753-767. Denton, E. J. and Shaw, T. I. (1961). The buoyancy of gelatinous marine animals. J . Physiol., Lond. 161, 14-15P. Denton, E. J. and Taylor, D. W. (1964).The composition of gas in the chambers of the cuttlebone of Sepia oficinalie. J . mar. bwl. Ass. U.K. 44, 203-207. Denton, E. J., Gilpin-Brown, J. B. and Howarth, J. V. (1961). The osmotic mechanism of the cuttlebone. J . mar. biol. Ass. U . K . 41, 361-364. Denton, E. J., Gilpin-Brown, J. B. and Howarth, J. V. (1967). On the buoyancy of Spir~la: spirula. J . mar. biol. ASS.U.K. 41, 181-191.

”.

”.

266

E. J. DENTON AND J. B. OILPIN-BROWN

Denton, E. J., Gilpin-Brown, J. B. and Shaw, T. I. (1969). A buoyancy mechanism found in cranchid squid. Proc. R. SOC.Lo&. B, 174, 271-279. Derham, W. (1726). “ Philosophical experiments and observations of the late eminent Dr. Robert Hooke, S.R.S.”. 391 pp. W. Derham, London. Diamond, J. M. and Bossert, W. H. (1967). Standing-gradient osmotic flow. A mechanism for coupling of water and solute transport in epithelia. J. gen. Physiol. 50, 2061-2083. Donovan, D. T. (1964). Cephalopod phylogeny and classification. Biol. Rev. 39, 269-287.

Flower, R. H. (1965). Saltations in nautiloid coiling. Evolution, L a m t e r , Pa. 9 (3), 244-260.

Flower, R. H. (1964). Nautiloid shell morphology. Memoir 13. State Bureau of mineral resources New Mexico Institute of Mining and Technology, Socorro, New Mexico. Furnish, W. M. and Glenister, B. F. (19640). Paleoecology. In “ Treatise on invertebrate paleontology Part K Mollusca ” (R. C. Moore, ed.). pp. 114124. University of Kansas Press and Geological Society of America. In Furnish, W. M. and Glenister, B. F. (1964b). Nautiloidea-Ascocerida. “ Treatise on invertebrate paleontology Part K Mollusca ” (R. C. Moore, ed.). pp. 261-277. University of Kansas Press and Geological Society of America. Gray, J. E. (1845). On the animal of Spirula. Ann. Mag. Izat. Hist. 15,267-260. Gregoire, C. (1962). On submicroscopic structure of the Nautilus shell. Bull. Imt. r. Sci. nclt. BeZg. 38 (49), 71. Gregoire, C. (1967). Sur la structure des matrices organiques des coquilles de mollusques. Bwl. Rev. 42, 663-688. Griffin, L. E. (1900). The anatomy of Nautilus pompilius. Mem. natn. Acad. S C ~8,. 103-197. Gross, F. and Zeuthen, E. (1948). The buoyancy of plankton diatoms : a problem of cell physiology. Proc. R . SOC.Lond. B, 135, 382-389. Haller, B. (1895). Beitrage zur Kenntnis der Morphologie von Nautilus pompilizca. Denkachr. med.-naturw. Om.J e w , 8, 189-204. (Also in Semon Forschungreisen Auatrdien u. malayischen Archipel. 5.) Heptonstall, W. B. (1970). Buoyancy control in ammonoids. Lethakz, 3,317-328. Hober, R. (1946). “ Physical chemistry of cells and tissues ”. 676 pp. J. and A. Churchill, London. Hoyle, W. E. (1886). Report on the Cephalopods collected by H.M.S. Challenger during the years 1873-76. Challenger Rep. 2001.16, 245 pp. Jacobs, M. H. (1940). Some aspects of cell permeability to weak electrolytes. Cold Spring H a r b . Symp. qwznt. Bwl. 8, 30-39. Kerr, J. G. (1931). Notes upon the Dana specimens of Spirula and upon certain problems of cephalopod morphology. Oceanogr. Rep. ‘Dam’ Exped. 1920-22, 8, 1-36.

Keynes, R. D. (1969). From frog skin to sheep rumen : a survey of transport of salts and water across multicellular structures. &. Rev. Bkphye. 2 (3), 177-281.

Krogh, A. (1939). “ Osmotic regulation in aquatic animals ”. 242 pp. Cambridge University Press, London. Lasker, R. and Theilacker, G. H. (1962). Oxygen consumption and osmoregulation by single Pacific sardine eggs and larvae (Sardinops cawulea Chard). J . Corn. int. Explor. Mer. 27, 26-33.

FLOATATION MECHANISMS IN MODERN AND FOSSIL CEPHALOPODS

267

Milroy, T. H. (1897). The physical and chemical changes taking place in the ova of certain marine teleosteans during maturation. Rep. Fish. Bd Scotl. 16, 135-152. Mutvei, H. (1964a). On the shells of Nautilus and Spirulrr with notes on the shell secretion in non-cephalopod molluscs. Ark. zool. 16, 221-278. Mutvei, H. (1964b). Remarks on the anatomy of recent and fossil Cephalopoda, with description of the minute shell structure of belemnoids. Stockh. Contr. Geol. 11 (4), 79-102. Mutvei, H. (1971). The siphond tube in Jurassic Belemnitida and Aulacocerida. Bull. gwl.Inat. Univ. Upsala, N.S. 3, 27-36. Mutvei, H. (1972a). Ultrastructural studies on cephalopod shells. Part I. The septa and siphonal tube in Nautilw. Bull. geol. Inatn Univ. Up&, N.S. 3 (8),237-261. Mutvei, H. (1972b). Ultrastructural studies on cephalopod shells. Part 11. Orthoconic cephalopods from the Pennsylvanian Buckhorn asphalt. Bull. geol. Inetn Univ. Upeala;, N.S. 3 (9), 263-272. 707 pp. Pitman, Nicol, J. A. C. (1960). “ The Biology of Marine Animals London. Owen, R. (1832). “ Memoir on the pearly Nautiluu (Nautilus pompdiua) 68 pp. Council of the Royal College of Surgeons, London. Phillips. J. (1866-70). ‘‘ A monograph of British Belemnitidm ”. London Palmontographical Society. Potts, W. T. W. (1965). Ammonia excretion in Octopue dofeini. Comp. Bwchem. Physwl. 14, 339-355. Potts, W.T. W. and Parry, Gwyneth (1964). “ Osmotic regulation in animals 423 pp. Pergamon Press, London. Pruvot-Fol, A. (1937). Remarques sur le Nautile. Extrait dea Comptea Renduu du X I I e Congrea International de Zoologie, Lisbonne (1935) 1652-1663. Raup, D. M. and Takahashi, T. (1966). Experiments on strength of cephalopod shells. Bull. geol. SOC.Am. (Program for 1966 Annual Meetings) 172-173. Robertson, J. D. (1949). Ionic regulation by some marine invertebrates. J. exp. BWl. 26, 182-200. Robertson, J. D. (1953). Further studies on ionic regulation in marine invertebrates. J. exp. Bwl. 30, 277-296. Roper, C. F. E. and Bnmdage, W. L. (1972). Cirrate octopods with associated deep-sea organisms : New biological data based on deep benthic photographs (Cephalopoda). Smithon. Contr. 2001.121, 1-46. Schindewolf, 0. H. (1968). Aspects of ammonoid paleontology. In “ Developments, trends and outlooks in paleontology (R. C. Moore, ed.) J . Paleont. 42 (6). 1365-1367. Schmidt, J. (1922). Live specimens of Spirula. Nature, Lond. 110, 788-790. Teichert, C. (1964). Morphology of hard parts. I n “ Treatise on Invertebrate Paleontology Part K , Mollusca 3. K13-K55. Teichert, C. (1967). Major features of cephalopod evolution. I n “Essays in Paleontology and Stratigraphy”. Special Publication no. 2, pp. 162-210, University of Kaiisas, Department of Geology. Teichert, C. (1968). Nonammonoid Cephalopoda. I n “ Developments, trends (R. C. Moore, ed.) J. Paleont. 42 (6). 1364and outlooks in paleontology 1366.

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”.

”.

268

1.J. DENTON A W D J. B. OILPIN-BROWN

Tormey, J. MOD.and Diamond, J. M. (1967). The ultrastructural route of fluid transport in rabbit gall bladder. J . gen. Phy8iol. 50, 2031-2060. Trueman, A. E. (1941). The ammonite body-chamber, with special reference to the buoyancy and mode of life of the living ammonite. Q. J Geol. SOC.96,

339-383. Westermann, G. E. G. (1971 Form,structure and function of shell and siphuncle in coiled mesozoic ammonoids. Contr. R. Ont. M u . , fife Sci. 18, 39 PP. Willey, A. (1902). “ Contribution to the natural history of the pearly Nautilus : A. Willey’s zoological results Vol. 6, pp. 691-830. Cambridge University F’ress, London.

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