Adhesion and substrate choice in mussels and barnacles

Adhesion and Substrate Choice in Mussels and Barnacles 1 D. J. CRISP, G. WALKER, G. A. Y O U N G , AND A. B. Y U L E

NERC Unit of Marine Invertebrate Biology, Marine Science Laboratories, University College of North Wales, Menai Bridge, GwyneddLL59 5EH, UnitedKingdom Received October 1l, 1983;accepted June 5, 1984 The force of adhesion of mussel byssus pads appears to be a function of the surface energy of the substratum, increasing with the polarity of the surface. Barnacles at the cyprid stage possess a mechanism for temporary adhesion by an antennulary attachment organ which can withstand a pull per unit surface area of several atmospheres. Associated with temporary adhesion is a capacity for conspecific recognition which can be simulated by spreading adsorbed layers of integumentary protein on a suitable solid surface. The mechanism of recognition is not fully understood and there remain problems in understanding why adsorbates, ubiquitously present in natural sea waters, or thick biofilms, do not appear to interfere with the tactile chemical sense. Both mussels and barnacles show a preference for adherends to which the natural cement will eventually form a stronger bond. © t985 Academic Press, Inc.

c o m m o n to many bivalves (10). In Mytilus it retains its function as an attachment organ throughout life. The large size and accessibility of the byssal attachment makes it very suitable for investigation. The byssus consists of stem, thread, and pad or plaque. The stem is rooted in the muscular tissues at the base of the foot. The numerous byssal threads arising from the stem consist of two parts. The proximal third consists of a pale elastic material. The distal part is of harder, darker protein, widely believed to be quinone tanned on the outside, with an inner core o f collagenous material. At its extremity the byssus thread expands into a pad which is moulded to the substratum and it adheres strongly. The pad is lanceolate in shape, variable in size, rubbery in texture, and polyphasic in structure, each phase having been formed by different glands in the foot. A thin layer of bioadhesive, probably containing phenolic groups, binds the pad to the substratum. When the pad is being secreted, the foot is active in pressing and smearing the semifluid precursor on to the solid surface. G. A. Young has found that the size o f the pad varies with that of the mussel. If log(area o f pad, a) is regressed on the length

1. I N T R O D U C T I O N

A sessile organism responds to the surface properties of potential substrata in two ways. First, it may choose it as a potential site or reject it. Secondly, if it is selected, the organism must adhere to it sufficiently firmly so that it can survive. It is these two processes alone that determine whether or not there is a fouling problem to contend with. Both are intimately concerned with the fundamental chemistry o f surface energy. In this paper we examine the exercise of choice and the subsequent adhesion to a variety o f surfaces on the part of two of the most important fouling organisms, mussels and barnacles. 2. THE MUSSEL MYTILUS EDULIS

2.1. Nature and Formation of the Byssus and Byssus Pad (Fig. 1) The byssus apparatus is a post larval fixation organ extending from the foot which is Presented at the symposium"Initial Eventson Bioattachment at the Solid-Liquid Interface," held at the American Chemical Societymeeting, Las Vegas,Nevada, March 198t, under the auspicesof the ColloidChemistry Division. 40 0021-9797/85 $3.00 Copyright © 1985 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 104, No. 1, March 1985

ADHESION IN MUSSELS AND BARNACLES

stem cuffs.~ thread~ proximalthread ~threodlength ofvariable !//,i [ 5/, .:l

,distalthread

41

pads being attached to slate than to glass, and more to glass than to paraffin wax. Table I shows that in five consecutive weekly trials, the n u m b e r of pads per unit area o f surface was always greater by an order of 3-4 times on the more polar surface. It should be noted that although the preference for slate and glass over wax was numerically similar, there remained a clear preference for slate over glass.

2.3. Force of Adhesion

FIG. 1. Diagram of mussel byssus apparatus.

of the mussel, l, the equation in millimeter units is found to be a = 0.00208l 1"83. The exponent does not differ appreciably from 2, showing that there is a roughly linear correspondence between mussel size and pad dimensions.

2.2. Choice of Substratum Mussels are by no means immobile; they can adjust their position by breaking byssus threads and reattaching new ones, or even by shedding the whole byssus complex and regenerating another. Young mussels are highly active, but mobility diminishes with age. When the complete set of byssus threads is formed, they anchor the mussel in all directions. However, pads are not laid down on a purely geometric design, the animal exercises choice in regard to the substratum to which each pad becomes attached. In an experiment in which mussels were offered a choice between two alternative substrata, arranged in chequerboard fashion, the n u m b e r of pads laid down were found to favor consistently one or other type of surface. O f three surfaces offered in paired trials, the order of choice was always the same, more

When pulled, the byssus thread m a y break, leaving behind the pad, or the pad itself m a y become detached in whole or in part. When the force of adhesion is very large, the threads almost invariably break, so that adhesion is underestimated. Even if those measurements in which the threads break are eliminated, the result in distribution of break loads would be biased toward lower values. Considerable improvement in pad detachment was obtained, however, if the distal end of the thread was reinforced by wire and "Superglue" before pulling. Three processes appear to be involved in failure (Fig. 2): (i) Breaking the bond between pad and surface (adhesive failure), (ii) peeling, and (iii) tearing and cohesive failure within the pad.

TABLE I Ratio of Numbers of Pads/Unit Area on Each of Two Surfaces Presented Simultaneously to Mytilus edulis for 1 Week Pairs of Surfaces Week

1 2 3 4 5 Geometric mean

Slate:glass Slate:wax Glass:wax

6.14 4.27 2.02 1.57 2.08 2.80

3.27 3.00 8.00 7.38 8.32 5.44

5.71 7.53 3.00 5.70 8.14 5.69

t = Mean/S.E. (log units) 4.0 7.5 9.9 P = Probability mean > 1.0 0.015 0.003 <0.001

Journal of Colloid and Interface Science, Vat. 104, No. 1, March 1985

42

CRISP ET AL. Q

Pad detachment .o.'e t-.-

~earing

AYl ~

Jc~

),~

~g 6.

FIG. 2. Processes operating during pad detachment. S

6 Log

Each measurement of break load was divided by a measurement of the area of adhesive failure obtained by examining remnants of the pad. Break loads measured in Newtons per square meter were arranged as cumulative frequencies of failure, transformed to probits and plotted against log break load. There was a considerable scatter of points about the regressions, especially at the low frequency, low break load region, where we consider that accidental defects and strains in the pad/thread system m a y have led to early failure. As can be seen in Fig. 3, the linearity was m u c h better in the high break load region, which involved well formed threads without serious defect, even though these constituted a minority. The results in Table II show that the pads adhere m u c h more strongly to polar surfaces such as slate and glass, which expose a preponderance of high energy, charged silicate and alumina residues against the proteinaceous adhesive of the pad. The adhesive force to the plastic acetal, which consists largely of alkoxy residues, was intermediate in strength, while adhesion to nonpolar materials, paraffin wax and polytetrafluoroethylene with low energy CH3 and CF 3 groups at the surface was so low that the pads usually detached whole and sometimes spontaneously. Surprisingly, the adhesion of pads formed during winter is significantly weaker than that of pads formed in summer, although the need for strength is greater during winter storms. Price (9) also noted seasonal differences but did not differentiate between the numbers of threads and their individual adJournal of Cblloid and Interface Science, Vol. 104, No. 1, March 1985

7

Force/unit area

FIG. 3. Plot of cumulative frequency of failure and derived probit against log(forceper unit area in N m-2) for byssus pads on glass in winter.

hesive strength. She noted that the mussel as a whole was less strongly attached in May than in September. Mussels are capable of producing threads more quickly in s u m m e r because of their higher metabolic rate, but the greater adhesive strength must be due to differences at the adhesive-substrate boundary. Possibly more silt is accidentally incorporated in winter, reducing the effective area of contact, possibly the adhesive material cures more rapidly and effectively at higher temperature.

TABLE II Results Giving Adhesion Force/Unit Area at 50% Failure Rate for Mytilus edulis ByssusPads Attached to Various Surfaces with Upper and Lower 95% Confidence Limits Taken from Regressionsof Probit on Log Force/ Area (except last entry) Force/area in l0 s N m -x Surface

Season

Slate

Summer Winter Summer Winter Winter Winter Winter

Glass Acetate PTFE Paraffin wax Paraffin wax (force/area regression)

Winter

Lower

Mean

Upper

8.4 4,9 6.7 2.8 0.44 0.12 0.12

8.5 5.6 7.5 3.2 1.12 0.13 0.13

8.6 6.4 8.3 3.6 2.88 0.14 0.13

-0.02

0.20

0.43

ADHESION

IN MUSSELS

AND

43

BARNACLES

T A B L E 111

Pad Area on Various Surfaces Surface

Mean areaa

Slate Glass

2.139 2.739

Mean polar 2 . 4 3 9

Paraffin wax

2.981 3.031

Mean nonpolar 3 . 0 0 6

PTFE

Analysis of variance Between surfaces Between individuals Residue Significant Pads on Pads on Pads on Pads on

Mean square

F

p

1.509 2.357

8.43 13.17

<0.001 <0.001

0.179

(1.00)

3 8 24

difference at P = 0.05 = 0.41 wax and PTFE not significantly different glass possibly different from nonpolar surfaces slate smaller than all other surfaces polar surfaces together smaller than on nonpolar

Nine individuals.

2.4. Pad Size and Shape

air/water interface. The globule of cement therefore covers a larger area on nonpolar As Table III shows, the area of the pad surfaces. We do not believe that this is an differs from one individual to another, but is adaptation to increase the area of adhesion, also significantly smaller on polar and larger compensating for a lower adhesive force, but on nonpolar surfaces. The appearance is also is rather an inevitable consequence of the different, those on high energy polar surfaces balance of surface energies predicted from having smaller contact angles 0 than those the Dupr~ equation. As we have argued on low energy surfaces (Fig. 4). Note that O elsewhere (l 1), if the solid surface is hydrois measured in the water phase from the phobic, it is easier for the bioadhesive to solid/water interface to the tangent to the displace water and spread over a larger area cement phase surface and increases with low- than would be the case for a hydrophobic ered solid surface energies as it would at an surface: COS 0 ~--" "YCS - - '~SL

Byssus threod Sea ~oter .L-

pa~ cement-c-

(G) High energy(gloss,slote)

(Dupr~equation).

~LC

(b) LOW energy(~ax~PTFE)

FIG. 4. Shape of pads and modification of contact angle on hydrophilic (left) and hydrophobic (right) surfaces.

If A Wsc and A ~VsL a r e increases in adhesive energies of the solid/cement and the solid/ liquid (water) interfaces, when going from less polar to more polar solid surfaces, and A0 the corresponding increase in contact angle, then the Dupr6 equation can be shown to reduce to N A p C O S 0 ----

NAp~sL

-- NApWsc 'YLC

Journal of Colloid and Interface Science, Vol. 104, No. 1, March 1985

44

CRISP ET AL.

where NApWsL is the increased energy of adhesion of water to a more polar surface NApWsc is the increased adhesion of cement to the solid, and 3'LC is the same for both surfaces. Hence NApO=

--NAp WSL "-~ NAp Wsc

3'LC Sin0

Since between 0 and 180 °, sin 0 is positive, 0 decreases as Wse increases from nonpolar to polar. Conversely, this analysis shows that 0 should increase as Wsc, the energy of adhesion of cement to solid, increases with the polarity of the surface. We know that eventually at least the adhesive force, which is almost certainly a measure of adhesive energy, is much greater at a polar surface. If this were true at the .outset it should have the effect of increasing contact angle 0 and assisting in the spreading of the cement. That this does not appear to take place may result from changes at the solid interface after the initial spreading of the bioadhesive. The latter is a viscous protein matrix containing polar molecules and side chains which may take time to diffuse into positions adjacent to the solid surface where its free energy would be reduced. Changes in surface energy embodying the same principle where organic substances were allowed to crystallize in contact with water rather than air have long been known (6). 3. THE BARNACLE B a l a n u s balanoides

3.1. Barnacle Adhesion Processes There are at least four distinct adhesion mechanisms occurring successively in the barnacle life history. (1) During its presettlement exploratory phase the cypris larva "walks" on its two antennules which adhere temporarily by attachment organs (Fig. 5). The surface applied to the substratum is covered by a dense cloak of minute cuticular hairs of dimensions 0.2 X 1 t~m. Opening on to this surface are sense organs which may be chemo- or mechanoreceptors, a large cement duct from the main Journal of Colloidand InterfaceScience,Nol. 104,No. t, March 1985

cement glands and small antennulary cement glands. Only the latter are likely to be involved in adhesion. (2) At fixation a relatively large volume of larval bioadhesive is discharged through pores in the attachment disc from the major cement glands on either side of the body. This large blob of cement usually embeds both antennules and the immediate region of the cypris ventral surface so preventing further translation. (3) Up to a week after metamorphosis, as the basal area of the "pinhead" barnacle becomes applied to the substratum it adheres by a mechanism not yet understood. When pulled off at this stage a force greater than that to remove the cyprid element is required though the cyprid cement and embedded structures are always left behind. (4) As the adult barnacle develops the secondary cement glands are formed, whose ducts open through the base of the barnacle. They spread rings of cement between the base and the substratum and supplant all the other mechanisms in the adult.

3.2. Choice of Surface The cypris stage of the barnacle has been the subject of intensive investigation on account of the refined sensory equipment by means of which it selects its habitat. The most challenging observations from the point of view of the solid-water interface concern the influence of adsorbed materials. It has been demonstrated that conspecific integumentary proteins (arthropodin) and related substances greatly increase settlement, thus encouraging gregariousness (1, 2). Monolayer quantities only appear necessary (3). Moreover the cyprid responds to these materials only when they have been adsorbed on a solid surface; they have no biological activity in solution. Two mechanisms for this tactile chemical sense have been suggested. Nott and Foster (8) showed that the antennulary attachment organ presents open-ended receptors thought

45

ADHESION IN MUSSELS A N D BARNACLES

$.r

a.d. c.

axial dome cuticle

m.t. p.g.

c.v. cm. cm.r. den. IV g. m.l.

cuticular villi cement duct radial canal of cement duct dendrites to 4th segment antennular gland longitudinal muscle

s.a. s.p s.r. set 1 set 2 vm.

transverse muscle pore of antenular gland axial sense organ postaxial sense organ radial sense organ preaxial seta postaxial seta velum

FIG. 5. The attachment organ o f the antennule of the cyprid B. balanoides reconstructed from sections. The internal structures are seen by cutting away the preaxial side, etc. The sensory organs are drawn in solid black. (Reproduced, by permission o f the publisher, from Nott and Foster (8), courtesy o f the Royal Society.)

to be chemosensory. Since the cyprid places the attachment organ on the substratum they suggested that the adsorbed protein layer might be broken down enzymaticany, the resulting amino-acid distribution being recognised by the normal chemical sense. Crisp

(4) however thought it more likely that the attachment organ's recognition might be analogous to that of antibody by antigen~ The physicochemical force developed by the matching of part of the antennulary surface to the monolayer surface might then be Journal of Colloid and Interface Science, Vol. 104,No. 1, March 1985

46

CRISP ET AL.

mediated by mechanoreceptors responding to the pull needed to remove the attachment organ from the surface. The fine hairs present would maximize contact area. Settlement stimulated by adsorbed protein layers is not a natural phenomenon; in nature the settlement response is elicited only b y arthropodins, probably in crosslinked form, on the barnacles' outer surface. Presumably the molecular pattern of adsorbed anthropodin is so similar to that of the natural integumental surface that it simulates the natural phenomenon. Thus although an unnatural system, monolayer experiments offer a very amenable model by which to study the phenomenon of tactile chemical sense. To demonstrate the response, it is necessary that the solid surfaces should be capable of adsorbing protein and that the appropriate arthropodin should be present there in the correct configuration. Only polar surfaces have been found to be consistently suitable for demonstrating the effect, for example, slate and phenolformaldehyde plastic. Low energy surfaces such as wax, polythene and PTFE are not suitable, nor is glass. Glass may be unsuitable because of its negative charge at the pH of sea water; since arthropodins have isoelectric points in the region of pH 4-5 they also would be negatively charged, reducing their energy of adsorption on glass. As Loeb and Neihof (7) have shown, all surfaces left in natural sea water acquire a film of macromolecules, while later quite thick microbial films form on them. Hence the animals are not choosing as in a laboratory between a chemically clean surface and one covered by a specific protein, but between unspecified natural films covering either an inert or a specific protein surface. This condition was simulated by adsorbing on slate a mixture of arthropodin with several times its own amount of gelatine, which is inactive in promoting settlement, and offering it to cyprids. The cyprids continue to recognize that arthropodin is present. Perhaps the arthropodin layer is selectively adsorbed and Journal of Colloid and Interface Science, Vol. 104, No. 1, March 1985

can still interact with the antennulary surface despite the presence of less strongly adsorbed molecules in the vicinity. Perhaps the cyprid cleans the adherent matter from the surface before testing it. More consideration of the behaviour of macromolecules at solid surfaces needs to be applied to this problem. In addition to the specific tactile chemical sense the cyprid exercises choice between the surfaces in respect of their curvature, texture, and surface energy (4). Concave surfaces are preferred to convex, rough to smooth and low energy surfaces are usually avoided. However our knowledge of how barnacles react to inert surfaces is somewhat deficient and probably varies from species to species.

3.3. Adhesive Forces during the Phase of Temporary Attachment A. B. Yule has shown that cyprids are sufficiently robust to withstand being fixed by cyanoacrylate glue to thin nichrome wires (Fig. 6). They will then attach by the antennules and attempt to walk on a submerged surface. If this surface is gently lowered the force at detachment can be recorded on a microbalance. Table IV records the forces obtained from a single uniform batch of cyprids for a variety of surfaces. The surfaces were neither uniformly smooth nor similar in color so it is not possible at this stage of our investigation to attribute differences wholly to those of surface energy. The order of magnitude of the adhesive force to the stronger adherends is quite high, clearly in excess of an atmosphere. This rules out the possibility that the antennular organ is attached by suction--a view previously dismissed on structural evidence by Nott and Foster (8). It will be seen that those substrata which develop low energies of adhesion are in general the smoother ones, for example glass and perspex. They also tend to be those with lower surface energies, though glass and polyvinyl chloride are not in the expected ranking order. However, glass is a particularly poor surface for attachment of this species, glass

ADHESION IN MUSSELS AND BARNACLES

47

nichrome wire

~thoraclc Iimbs

"Superg]ue"4 compounde J

~1~ ant u1e e)//p//,/~n slate--//~

///

///////i/

(a)

(b)

~"Superglue" excisedontennu]e #

//// (c]

FIG. 6. Attachment of cyprids to nichrome wire for measurement of the force of temporary adhesion.

vessels being widely used for preventing the animals from settling. Black matt polyvinyl chloride is known in practice to be a good settlement surface. Hence the order of adhesive forces developed during temporary adhesion appeared to correlate reasonably well with surface choice at settlement. Figure 7 records a series of experiments in which the force developed on rough slate was compared with the same surface coated with conspecific arthropodin. It will be seen first that the adhesion increases throughout the season reaching a m a x i m u m when settlement is just past its peak. Since it is known that

the tendency of cyprids to settle increases during the season and with age, the seasonal change suggests that some degree of voluntary control m a y be involved in the force of detachment. The upper line in Fig. 7 shows that the force developed in parallel tests on slate treated with arthropodin is invariably greater than on untreated slate. Although the increase is not dramatic, it is highly significant. Unfortunately, the excised antennule (Fig. 6) could not be made to attach, the musculature and nervous coordination of the intact animal is evidently necessary to obtain good adhesion. It will be noticed that, al-

TABLE IV The Temporary Adhesion of B. balanoides Cyprids to Different Substrata

Mean removal force (units of 105 Nm-2)

Beeswax (smooth and translucent)

Glass (smooth and transparent)

Perspex (smooth and transparent)

0.66

0.68

1.11

Smooth slate (smooth grey)

Reinforced phenolforrnaldehyde Tufnol (matt brown)

Polyvinyl chloride Darvic (matt black)

1.26

1.45

1.701

Analysis of variance a Source of variation

Between treatments Within treatments

~(1'

Mean square

1:

P

5 54

1.7379 1.7341

10.022

<0.001

At P < 0.05, mean removal force on (beeswax = glass) < perspex < (slate = Tufnol = Darvic) (by NewmanKeuls test). Journal of Colloid and Interface Science, Vol. 104, No. 1, March 1985

48

CRISP ET AL. 3X10 s

"

in" c o o t e d s l o t e

2xlO 5

E C|egn s l o t e

ixlO 5

ADrJl

MOy

FIG. 7. Increased force required to detach the antennulary organ of B. balanoides when slate surfaces are coated with conspecificarthropodin (spring 1979). Upper Line: arthropodin treated slate, Lower line: clean slate, Vertical bars: standard error of the mean. though the forces developed can reach nearly three atmospheres, the cyprid can voluntarily remove the antennule from the surface. This phenomenon needs careful investigation, but we believe it is achieved by a process of peeling for which the musculature and joints appear to be adapted.

3.4. Adhesive Forces after Metamorphosis Table V records the force per unit area needed to detach B. balanoides at various ages. The observations were obtained as a result of various techniques, which need not be gone into here, devised to measure the maximum force to pull the barnacle from its

substratum. The animals were almost invariably attached to a standard semi-rough slate surface. When only part of the adhesive bond was found to be broken this was taken into account by measuring the area of adhesive or cohesive failure. As would be expected, the force increases rapidly with the size of the barnacle though the force per unit area rarely falls below or exceeds the range of 1 to 10 atm. It reaches its m a x i m u m of nearly 10 × 105 N m -2 (10 atm) when the cyprid has just attached permanently prior to metamorphosis. At this juncture, a very small area of cement has the task of retaining a relatively large and otherwise unattached body and so requires strong

TABLE V Forces of Adhesion in Acorn Barnacles (B. balanoides) Age or stage

Adhesion

Exploring c y p r i d

Temporary attachment

Cyprid fixation Pinhead, 1 week Juveniles, 2 months Adult, 5-10 months

Primary fixation Basal attachment Secondaryfixation Secondary fixation

Journal of Colloid and Interface Science. Vol. 104, No. t, March 1985

Failure

Microtrichiaand secretion Primary cement ? Secondary cement Secondary cement

Force to detach barnacle

Force of adhesion to slate (N m -2)

20-50 nag

1.5-3 × 105

1.5 g 6g 600 g 5-20 kg 5-10 kg

9.7 × 105 0.64 × 105 1.2 × 105 4.9 × 105 1.5-2 × 10* (Glass: B. crenatus)

ADHESION IN MUSSELS AND BARNACLES adhesion. The secretion from the cyprids' cement glands possesses all the required components for tanning--protein, polyphenol oxidase, and polyphenol--and the resulting strongly adherent material with the antennules embedded in it is characteristically left whenever barnacles are removed. Darwin (5) frequently figures these traces below the barnacle base. The basal attachment of 1-week-old individuals is probably important in the transition between the larval and adult mechanisms but appears to involve no specific cement and exhibits the least adhesive force/area. The adult's secondary cement, like the cyprid's is proteinaceous. Cohesive failure within the cement usually takes place and can be seen by staining the cleaved surfaces with bromphenol blue. When adult barnacles are pulled they rarely come away whole, generally part of the base is left behind. Figure 8 illustrates our results for adult barnacles showing the regression of force on area of cement cleaved. The regression line did not pass through the origin; a force requirement was present even if the cleaved area were extrapolated to zero. This intercept was quite significant and is attributed to the

lOOOO

8000

6000

4000

2000 ..

0,0

0.2

0.4 0.6 0.8 AREA OF CEMENT (cm2)

1,0

FIG. 8. Force (in g) to detach adult B. balanoides regressed on area o f cement cleaved (in cm2). The slope is 4.9 kg cm -2, 4.9 × 105 N m -2, or 4.9 atm.

49

force required to tear the base. Separate regressions on barnacles of different size revealed that the larger animals had significantly larger intercepts. These measurements were made on B. balanoides, a barnacle with a membraneous base. Similar observations on B. crenatus which has a calcareous base resembled those of Fig. 8 and had almost equal slope in the region of 5 × 105 N m -2. This suggests that the cohesive strength o f the cement was similar in both species and that the calcareous base had little influence on its adhesion to the substratum. However, when B. crenatus was attached to smooth glass, lower slopes o f only 1.5-2 × 105 N m -2 were obtained. It must be concluded that the adhesive force on glass is less than that on slate so that with glass cohesive failure was accompanied by adhesive failure. This appeared to be the case from the appearance of areas of adherent cement on the detached base. 4. C O N C L U S I O N S

Both in mussels and barnacles the choice between inert surfaces tends to favor those with higher critical surface energy. Although the ranking order of preference is not identical for barnacle cyprids and for the mussel byssus, for each organism there is a reasonably close correspondence between those surfaces chosen for attachment and the adhesive force per unit area which we have measured. In mussels the mechanism by which selection is made is not as yet understood. In a cyprid we suggest that the stimulus to settle may be mediated by registering the force of adhesion of the surface to the temporary adhesive employed during exploration. If this material resembled in its adhesive properties the bioadhesives subsequently used by the animal, the correspondence between substrate choice and adhesion would be readily explicable. It is clearly advantageous for a mussel to choose surfaces to which the byssus adheres more strongly. In barnacles and other invertebrate larvae, the choice is complicated by Journal of Colloid and Interface Science, Vol. 104, No. 1, March 1985

50

CRISP ET AL.

factors other than the potential strength o f the adhesive bond, such as the necessity to settle near conspecifics and to choose ecologically desirable habitats. H o w e v e r a n y substratum to which the animal could b o n d only weakly would be disadvantageous however desirable it might be in other respects. H e n c e surfaces to which the protein-based bioadhesives are unlikely to f o r m strong bonds should be recognized as unfavorable and avoided. REFERENCES 1, Crisp, D. J., and Meadows, P. S., Proc. R. Soc. London Set. B 156, 500 (1962). 2. Crisp, D. J., and Meadows, P. S., Proc. R. Soc. London Set, B 158, 364 (1963).

Journal of CoUoid and Interface Science, Vol. 104, No. 1, March 1985

3. Crisp, D. J., in "Proceedings, Fifth Marine Biological Symposium, Gotebrrg, 1964," pp. 51-65 (1965). 4. Crisp, D. J., in "Chemoreception in Marine Organisms" (P. T. Grant and A. M. Mackay, Eds.). Academic Press, New York, 1974. 5. Darwin, C., "A Monograph on the Sub-class Cirripedia II the Balanidae, the Verrucidae, etc." Ray Soc., London, 1854. 6. Devaux, H., J. Phys. Radiat. 4, 184 (1923). 7. Loeb, G., and Neihof, R. A., Adv. Chem. Ser. 145, 319 (1975). 8. Nott, J. A., and Foster, B. A., Philos. Trans. R. Soc. London Ser. B 256, 115 (1969). 9. Price, H. A., J. Mar. Biol. Assoc. U. K. 60, 1035 (1980). 10. Yonge, C. M., J. Mar. Biol. Assoc. U. K, 42, 113 (1962). 11. Young, G. A., and Crisp, D. J., "Adhesion 6," Chap. 2. Academic Press, New York, 1981.