The burrowing activity of Nephtys cirrosa Ehlers (Annelida: Polychaeta)

J. exp. mar. Biol. Ecol.,

1976, Vol. 24, pp. 307-319;

THE BURROWING

ACTIVITY

(ANNELIDA

@ North-Holland

OF NEPHTYS

Company

Publishing

CZRROSA Ehlers

: POLYCHAETA)

J. H. TREVOR ZoologyDepartment,

The University,

Manchester,

England

Abstract: The digging activity of Nephtys cirrosa Ehlers is described and the major events in a representative ‘digging cycle’ outlined. The resting pressure or pressure maintained in the anterior coelom of an inactive worm by muscle tonus is 0.245 kPa whilst eversion of the proboscis once per cycle occurs with a major pressure pulse of 2-12 kPa. These pulses may be followed by a much lower amplitude pressure pulse as the tail is drawn along the burrow. Simultaneous recordings of pressure changes in the large anterior coelom and in the posterior septate part of the coelom indicate that the two regions are usually isolated during eversion of the proboscis: however, under certain circumstances, the pseudosepta may remain open so that the coelom may function as a single fluid-filled chamber. The activity of Nephtys during burrowing is compared with that of Arenicola and Sipunculus. Arenicola uses progressively more low-pressure scraping movements with its proboscis in each digging cycle as burial proceeds. This contrasts with a single powerful eversion of the proboscis in each cycle characteristic of Nephtys and Sipancrdrrs.

INTRODUCTION

The burrowing mechanisms of a number of relatively smooth-bodied vermiform animals have recently been investigated, for example that of Po/yphysiu (Elder, 1973) Lumbricus and Arenicola (Seymour, 1969, 1971) and summarized by Trueman (1975). Apart from a preliminary description of the burrowing process of Nephtys by Clark & Clark (1960), there is, however, little available information concerning the burrowing of a worm possessing well-developed parapodia, and this study has been undertaken in an attempt to rectify this. In common be divided through through

with most soft-bodied

into two parts,

the substratum. the water and

the initial

animals

the burrowing

penetration

process of Nephrys may

and the subsequent

progression

Initial entry may be effected either by swimming rapidly approaching the substratum obliquely, or by undulatory

swimming movements leading to sand being swept over the dorsal surface of the worm (Clark & Clark, 1960). The latter method was not observed in the present study. After initial penetration to gain lateral anchorage, burrowing involves rapid eversion of the proboscis into the substratum, punching a hole into which the worm may crawl. The principal musculature of the anterior part of the body of Nephtys (Fig. 1) consists of dorsal and ventral longitudinal muscle blocks, inter-segmental dorsoventral muscles, and oblique muscles running from the nerve cord to the body wall or the parapodia. The first 35 segments are aseptate and contain the proboscis appara307

J. W. TREVOR

308

tus in what may be termed the anterior coelom, while posteriorly the true septa have been reduced to gut suspensory muscles and ‘pseudosepta’ have developed from hypertrophied oblique muscles. Clark (1962) considers that the pseudosepta, which are incomplete around the gut, can be closed in order to isolate each segment hydrostatically.

I

RM

G

Ph

BT

DLM

/

I MM

PM

VLM

VNC

PS

OVM

Fig. 1. Stereogram to show the anterior coelom of Nephtys and some of the muscles used in burrowing: the anterior dorso-ventral muscles, the oblique muscles, including the extrinsic parapodial muscles, and the gut suspensory muscles are omitted: BT, buccal tube: DLM, dorsal longitudinal muscle; DVM, dorso-ventral muscle; G, gut; MM, motor muscles; Ph, pharynx; PM, protractor muscle running to anterior body wall; PS, first pseudoseptum following aseptate region; RM, radial muscles around mouth; VLM, ventral longitudinal muscles; VNC, ventral nerve cord: (constructed from Clark & Clark, 1960 and Dales, 1962).

MATERIALS AND

METHODS

Nephtys cirrosa Ehlers was collected as required from either the sand flats near

Llandudno, North Wales, or from Kames Bay, Isle of Cumbrae, and kept in shallow sand and aerated sea water at 12”42 “C in the laboratory. The length of the worms used was between 5 and 8 cm, the wet and dry weights of a 7.0 cm worm being R 4.50 mg and 85 mg, respectively. Recordings of the coelomic pressure were made using a Statham P.23 BB pressure transducer connected to the worm by flexible polythene tubing and a recurved hypodermic needle of 0.3 mm bore. Burrowing movements were observed directly or recorded by cinematography for subsequent analysis by placing the worms on the sand near to the wail of a Perspex tank. Observation was also facilitated by the use of powdered cryolite (Ward’s Natural Science Establishment Inc., Rochester, N.Y.) as a substratum. This has the same refractive index as sea water and a similar specific gravity to that of sand grains (Josephson & Flessa, 1972). EXPERIMENTAL THE MECHANISM

OBSERVATIONS

OF BURROWING

~ep~ry~ burrows rapidly and then may continue to move through the sand; in the descriptions of burrowing which follow, the term ‘digging period’ refers to the time

BURROWING

interval

between

moves beneath cycles’,

of burial

the substratum.

and subsequent

309

and the moment

when the tip of the tail

The digging period is composed

or steps into the sand,

proboscis

a

the beginning

IN NEPHTYS

each of which consists

movement

S

of a series of ‘digging

of a single eversion

of the worm forwards. E

About

of the

12 digging cycles

L

107

kPa

O-

i

1 1

I 1 I I

I

I

I

i

I

I

I

I

I

I

1 I

I ~-rt-l--~~

seconds

seconds Fig. 2. a, pressure pulses generated in the anterior coelom of Nepht.vs during burial: b, pulses recorded after burial when burrowing activity occurs in bursts: on the latter trace the ‘steps’ at eversion are more exaggerated than usual, resulting in a double peak: E, appearance of proboscis; L, posterior longitudinal contraction: S, undulatory swimming movements.

are usually necessary for burial and a high amplitude pressure pulse, associated with proboscis eversion, is generated in the anterior coelom during each cycle (Fig. 2a). The sequence of events in later cycles differs slightly from that in the initial ones, but the cycles subsequent

to the tenth may be taken as representative;

such digging cycles may be summarized (i) Variable period of immobility. (ii)

The pharynx

moves

forward

with reference within

the main stages of

to Fig. 3.

the coelom,

segments

15-50

begin

to

advance and the pressure in the anterior coelom begins to rise from its resting level; segments l-l 5 thicken dorso-ventrally and their parapodial rami diverge. (iii) Segments 15-50 continue to be drawn anteriorly, the proboscis begins to evert and there is a step on the pressure trace before it rises to a peak (Fig. 2a, Fig. 3). (iv) The proboscis becomes fully everted as the pressure in the anterior coelom falls. (v) The proboscis retracts and the pressure returns to near the resting level. (vi) The anterior segments move forward into the cavity in the sand. (vii) (L a t er c y c Ies only.) The posterior segments are pulled along the burrow by longitudinal contraction, usually accompanied by a small pressure pulse.

seg. lllllllli

no.

1111lrlllIIIIIIIII1IIIIIIIlllllllllllllillrllilrllllllll

I

T

14ITi

iv

vi

vii

kR

0

h ,:’ Ji ttt I

ii

I

I v vi vii

iii

8s

1

Fig. 3. Schematic diagram relating the stages of a late digging cycle (cycle 12) to the pressure pulses generated in the anterior coelom (below): diagrams are from the lateral aspect: the longer the vertical line on each segment the greater the parapodial anchorage (for further information see text p. 3091.

BURROWING

Dales (1962) described smaller protractor

how contraction

muscles is coordinated

311

IN NEPHTYS

of the “motor” with contraction

muscles

(Fig.

I ) and the

of radial muscles around

the mouth to enable the proboscis to be everted under fluid pressure; Clark & Clark (1960) had shown the latter to be necessary for eversion since Nephtys was unable to evert the proboscis

with an open cannula

inserted

into the anterior

coelom.

ft is

probable, therefore, that during the first part of stage (ii) of the digging cycle the motor muscles are contracting in order to draw the proboscis forwards. During the later part of stage (ii) and during stage (iii) contraction of the anterior longitudinal muscles, probably assisted by tension in the dorso-ventral muscles, produces the increase in pressure necessary for eversion (Clark & Clark, 1960) and draws segments 15-50 forwards. The parapodial anchorage of the first 15 segments is augmented both by the parapodia and by the increase in the dorso-ventral dimensions of later segments after they have been drawn anteriorly (Fig. 3).

Fig. 4. Dorsal

view of the anterior of Nephtys to show proboscis eversion peristaltic movements: traced from film.

and subsequent

retrograde

During stage (i) when the worm is immobile, the resting pressure, i.e., the pressure maintained in the coelom by muscle tonus alone, is 0.24.5 kilopascals (kPa) (about 2-5 cm water). The pressure pulses associated with proboscis eversion range in

J. H. TREVOR

312

amplitude

from 2-12 kPa, reaching

As the internal shortly

by retraction

proboscis

of the proboscis

when it is fully extended

tion is effected principally completed

a high level after some 6 digging cycles (Fig. 2a).

pressure falls off, maximal

the posterior

eversion

to be followed

muscles project

and Dales (1962) suggests that proboscis

by the contraction segments

(stage iv) is attained,

(stage v). The motor

remain

of these muscles.

stationary

When

into the retrac-

retraction

while those anterior

is

to segment

35 extend forwards into the cavity left by the proboscis (stage vi), either by lateral undulatory movements of the anterior segments, or by peristaltic movements (Fig. 4). Although Clark & Clark (1960) considered that changes in segmental length were chiefly compensated by changes in height, during peristalsis the width of the segments may decrease by 37.8 fi; relative to their maximum observed dimensions.

Proboscis ever&n:

sag. 70

seg. 70

1 \

1

\seg. 1

seg. 1

\

5s

5s

Fig. 5. Diagrams to illustrate the progression of Nephtys (from observations) in relation to the pressure pulses in the anterior coelom and eversion of the proboscis, during digging cycles from early (a) and late (b) parts of a digging period: S, undulatory swimming movements.

As the first 50 segments

move forwards

at eversion

the more posterior

segments

are subjected to longitudinal tension due to the muscular forces tending to move them forwards and the frictional forces between the tail and the burrow walls. When less than half of the worm is buried, during cycles 1-6, there is little resistance to forward movement of the tail and contraction of the anterior longitudinal muscles alone is sufficient for progression, this occurring during eversion of the proboscis (Fig. 5a). When, however, the worm is more deeply buried, further contractions of the posterior longitudinal muscles (or, occasionally, low amplitude, short-wavelength undulatory movements) are necessary to draw the hind end along the burrow (stage

BURROWING

vii) (Fig. 5b). Consequently, during RATE

IN NEPHTYS

313

the digging cycles show a marked

the latter part of the digging period

increase

in duration

(Fig. 2a).

OF BURIAL

The first 10 digging

cycles usually

occur at a rate of one cycle every l-2 set, and

the two or three subsequent cycles which are usually sufficient to complete burial at a slower rate of one every 4-8 set (Fig. 2a). About 10 min after burial the digging activity tends to recur in bursts of 4 or 5 digging cycles, each burst separated from the next by intervals of a minute or more (Fig. 2b).

Fig. 6. The rate of progression into the substratum as recorded by a movement transducer attached to segment 40 of Nephtys: initial penetration by indulatory swimming movements (S) and the first 4 or 5 digging cycles not recorded.

By attaching a movement transducer by thread to the anterior segments of a specimen of Nephtys which is then allowed to burrow, a step-like trace is produced which gives a convenient pictorial representation of the frequency of the digging cycles and burrowed

the distance during

(consisting

the digging

period

of both

vertical

(Fig. 6). The initial

and horizontal

components)

rate of progression

is rapid

but gradually falls off as the digging period progresses, mainly because of the decrease in the digging cycle frequency rather than a decrease in the distance burrowed per cycle. The apparatus

may be calibrated

so that the distance

burrowed

digging cycle may be measured

directly from the trace. The distance

burrows

reduced

per cycle is, however,

by the frictional

during

each

which the worm

forces and the inertia

of the

apparatus and so the following method of estimating a mean value was preferred. First, the length of the worm was measured and then the time and the number of digging cycles necessary for the worm just to bury itself noted. Only the second part of the burrowing activity was considered, the initial penetration by swimming movements being ignored. If Nephtys is removed from the sand immediately it has buried and is then replaced on the surface of the substratum it will attempt to bury itself again, although if this is repeated then tactile stimulation may be required for the initiation of the third and subsequent burials. The mean rates of burial during

J. H. TREVOR

314

5 trials are shown in Fig. 7a. The rates of progression a significant

decrease in the rates between

Trials

were very variable

1 and 2 and between

but there was Trials 2 and 3

(P < 0.01 and P < 0.001 respectively), although the differences in rate between Trials 3, 4, and 5 were not significant at the 10 y/, level. By monitoring the pressure pulses produced

in the sand

at proboscis

a Mean rate of progression

eversion

without

interfering

with

the worm,

b Mean cycle frequency

c Mean progression per cycle

14

‘+-+--I

L.1

0,

0.1

\ :m.s-’

cm. cycle-l

cycles.si

JI ..

5

0

5

0

5

Burial No. Fig. 7. In successive burials the mean rate of progression shows a significant decrease over the first three trials (a), apparently due more to a reduction in the mean frequency of the digging cycles (b) rather than to a reduction in the mean progression per cycle (c); latter calculated from the length of worm buried divided by the number of cycles necessary: graphs plotted on semi-logarithmic coordinates; means of 20, 17, 15, 9, and 4 movements for burials l-5, respectively: means LS.E.

it was found that the decrease in the speed of burial in successive trials was largely due to a reduction in the mean digging cycle frequency (Fig. 7b), which decreased by about 50 % between distance between

each of the first three trials,

burrowed trials.

FUNCTIONS

per cycle

(Fig.

rather

7c) which

than to a reduction

only

decreased

in the mean

by about

IO-15 g/,

OF THE PSEUDOSEPTA

Clark (1962) was of the opinion that the pseudosepta of Nephtys are closed during swimming movements. The function of a hydrostatic pressure barrier is important because contraction of the longitudinal muscles, during lateral undulation, can exert a tensile force via the coelomic fluid on the contra-lateral longitudinal muscles, and so lengthening the opposite side of the segment. The pressure in each segment would also prevent deformation of the parapodium, by its very turgidity, during the power

BURRO WING IN NEPHT YS

315

stroke. Clark & Clark (1960) are also of the opinion that the aseptate anterior 35 segments are isolated from the rest of the body during proboscis eversion. They obtained some evidence for this hypothesis by injecting a solution of methylene blue into the anterior eversions

coelom

and observing

or after wriggling

that it was confined

there until after several

by the worm. b

a

Seg. 80

seconds

seconds

Fig. 8. Simultaneous pressure recordings from aseptate (anterior) and septate parts of the coeiom during burrowing activity: a, usual type of record showing little or no pressure transmission through the coelom during burrowing activity: b, anomalous trace of a type occasionally encountered in which pressure pulses occur simultaneously throughout the coelom, although their amplitude is much reduced posteriorly: flat tops to peaks due to limit of pen travel: A, tactile stimulation; E, proboscis eversion; S, undulatory swimming movements.

Simultaneous pressure recordings from the aseptate (anterior) and the septate regions of the coelom during burrowing activity confirm that the anterior coelom is hydrostatically isolated from the more posterior segments during eversion (Fig. 8a). After tactile stimulation of the worm at points A to initiate burrowing it may be seen that the small pressure rises associated with undulatory movements (S) are present in both the anterior and the posterior coelom. The fact, however, that the eversion pulses (E) are only present anteriorly supports the findings of Clark & Clark and suggests that the similarity of the pressure rises is because both regions are engaged in similar undulatory activity rather than that the pseudosepta are open. Nevertheless, a small number of anomalous traces were obtained (Fig. 8b) in which high pressure pulses occur simultaneously in the anterior and posterior parts of the coelom. Many traces from both early and late stages of burrowing have been obtained. most of which demonstrate the isolation of the anterior coelom from the posterior, even from segment 47, about 12 segments behind the aseptate region: one or two

316

J. H. TREVOR

records do, however, indicate that there is some communication and this suggests that Nephtys may be able to control the pseudosepta and so restrict or enlarge the effective volume of the anterior coelom. DWX.JSSION

Clark (1962) suggested that it is an advantage for the segments of Nepht~~sto be hydrostatically isolated during undulatory movements. Water-tight septa do not, however, appear to be absolutely necessary for this method of locomotion since the aseptate anterior of the worm may progress by sinusoidal waves into the cavity formed by the proboscis. It is necessary that the anterior segments be aseptate to accommodate the retracted proboscis, but if the whole of the coelomic cavity were aseptate not only would the undulatory movements presumably be less efficient (by the reasoning of Clark, Ior. cit.) but also the pressure pulses which are necessary for eversion of the proboscis would have to be both generated and maintained by the entire length of the body musculature. These difficulties are resolved if the posterior of the worm is septate and is isolated from the anterior coelom which contains the proboscis. The possession of septa posteriorly could also permit some division of labour along the length of the worm since different pressures can be maintained in various parts of the body (Newell. 1950). This may only occur to a limited extent in Nephtys because in the early part of the digging period there are usually no undulatory movements of the posterior during proboscis eversion. Why are the pseudosepta incomplete around the gut in Nephtys? Their method of origin by hypertrophy of oblique muscles is probably important, but it is likely that there are aiso functional reasons. Mettam (1967) has suggested that the pseudoseptum may aliow free movement of the intestine at eversion w>hilestill acting as a pressure barrier if the gut is of regular dimensions. Most of the pressure records obtained simultaneously from both the anterior and the posterior coelom confirm this (Fig. 8a), one record showing no detectable pressure transmission even at segment 47, i.e.. 12 segments behind the aseptate region. In these circumstances only the musculature of the anterior region need maintain the pressure necessary for eversion. Nevertheless, the closure of the pseudosepta would imply a reduction in the total volume of coelomic fluid available to the anterior coelom. On occasions, however, some transference of pressure pulses to the posterior was detected (Fig. 8b). and although this would necessitate the high eversion pressures bzing maintained by a greater length of the body musculature, the effective volume of the anterior wouId be augmented and the change in volume of the anterior fluid system as the proboscis everts would be relatively much smaller. Additional turgidity of the head region consequent on the enlargement of the anterior coelom by patent pseudosepta could improve the anchorage of the worm and the penetration of the proboscis. It could thus be advantageous during burrowing through more compacted substrata, despite being more expensive energetically. The possession of pseudosepta capable of being

BURROWING

IN NEPHTYS

3t7

opened or closed may thus enable Nephtys to use whichever method of burrowing is most appropriate. a. Nephtys anterior coelom

b Si~nculus coelom

c Arenicola trunk coelom

d Ensis haemocoele

Fig. 9. Comparison of the types of pressure changes characteristic of the digging cycles of four softbodied burrowing invertebrates to show the points at which anchorages are formed and where probing occurs: E, proboscis eversion; PA, penetration anchor; TA, terminal anchor; p, probing of foot.

The digging cycles of ~e~~~y~, and their associated pressure pulses, invite comparison with other soft-bodied burrowing invertebrates. Very similar in many respects is Sipunculus (Trueman & Foster-Smith, 1976) (Fig. 9). In both animals there are two main pressure pulses per digging cycle, a major pulse associated with proboscis eversion and a single minor pulse, associated in SipuncuZus with fast proboscis retraction and the formation of a ‘terminal anchor’ (see Trueman, 1975) in order to draw the tail forwards, and in Nephtys associated with the tail being drawn along the burrow. Prior to the rapid eversion of the proboscis both animals form a ‘penetration anchor’ (Trueman, 1975), provided mainly by the parapodia in Nephtys (Fig. 3) but by thickening of the anterior in Sipunculus; to some extent these also function as terminal anchors. After eversion, Sip~~c~l~s forms a further terminal anchorage by dilation of the everted proboscis in order to draw the anterior of the

J. H. TREVOR

318

worm forwards anchor

by means of the proboscis

is necessary

retractor

since the head progresses

utilizing the parapodia as points d’uppui. The lugworm Arenicolu also shows major digging cycle (Fig. 9~); however, in contrast

muscles.

by undulatory and minor

In Nephtys no terminal or peristaltic

pressure

movements

pulses during

each

to Nephlys and Sipunculus, it is the major

peak that is associated with the formation of a terminal anchor and longitudinal shortening of the posterior trunk, although the magnitude of the peak (about 15 kPa) may be to aid active enlargement of the burrow rather than for anchorage alone (Seymour,

1971); the minor

pulses are associated

with proboscis

eversion

and the

erection of flanges on the first three segments to form penetration and terminal anchorages. A high pulse is not necessary for eversion of the proboscis since the buccal mass and pharynx are used in a scraping action rather than in the forceful thrusting manner of Neplzrys and, like many other burrowing invertebrates such as the bivalved mollusc Ensis (Trueman, 1975) (Fig. 9d), the sand is probed more times per cycle as the lugworm burrows deeper and encounters more compacted substrata (Seymour, 1971). Nephtys and Sipuncuhs on the other hand use only one probe, i.e., proboscis eversion, in each digging cycle, even in the later stages of burrowing, so that in Nephfys the reduction of the digging cycle frequency as the digging period progresses One important

must be due to other factors. reason for the decrease is the extra longitudinal

muscle contrac-

tion which is necessary for the drawing of the tail along the burrow during the later stages of the digging period (Fig. 2). Fatigue is also a major factor, as shown by the significant increase in the lengths of successive digging periods when the worm is encouraged to burrow repeatedly (Fig. 7). During only the second or third successive digging periods the worm may cease burrowing, either before or after the posterior longitudinal contraction of the current cycle, with a centimetre or so of its tail still protruding from the sand. This appears to be a disadvantage from the point of view of predation, although it is conceivable that the tail of Nephtys, being readily autotomised, is relatively expendable as is the case with Arenicolu marina (Wells, 1966). ACKNOWLEDGEMENTS

This work was done during the tenure of a N.E.R.C. Research Studentship. I am particularly grateful to Professor E. R. Trueman for much helpful criticism, to Mrs. K. Perry and Mr. L. Lackey for graphic and photographic assistance and to the Director and Staff at the University Marine Station for their hospitality during my visits to Millport. REFERENCES R. B., 1962. On the structure and functions of the polychaete septa. Proc. rool. Sot. Lond., Vol. 138, pp. 543-578.

CLARK,

BURROWING

IN NEPHTYS

319

R. B. & M. E. CLARK, 1960. The ligamentary system and the segmental musculature of Nephtys. Q. Jl microsc. Sci., Vol. 101, pp. 149-176. DALES, R. P., 1962. The polychaete stomodaeum and the inter relationships of the families of polychaetes. Proc. zool. Sot. Lond., Vol. 139, pp. 389428. ELDER, H. Y., 1973. Direct peristaltic progression and the functional significance of the dermal connective tissues during burrowing in the polychaete Polyphysia crassa (Oersted). J. exp. Biol., Vcl. 58, pp. 637-655. JOSEPHSON, R. K. & K. W. FLESSA, 1972. Cryolite: a medium for the study of burrowing aquatic organisms. Limnol. Oceanogr., Vol. 17, pp. 134-135. METTAM, C., 1967. Segmental musculature and parapodial movement of Nereis dirersicolor and Nephtys hombergi (Annelida: Polychaeta). J. Zool. Lond., Vol. 153, pp. 243-275. NEWELL, G. E., 1950. The role of coelomic fluid in the movements of earthworms. J. exp. Biol., Vol. 27, pp. 110-121. SEYMOUR, M. K., 1969. Locomotion and coelomic pressure in Lambricus terrestris L. J. exp. Biol., Vol. 51, pp. 47-58. SEYMOUR, M. K., 1971. Burrowingbehaviour in the European lugworm Arenicola marina. (Polychaeta, Arenicolidae). J. Zool. Lond., Vol. 164, pp. 93-132. TRUEMAN, E. R., 1975. The locomotion of soft-bodied animals. Edward Arnold, London, 200 pp. TRUEMAN, E. R. & R. L. FOSTER-SMITH, 1976. The mechanism of burrowing of Sipunculus nudus L. J. Zoo/. Lond., Vol. 179, pp. 373-386. WELLS, G. P., 1966. The lugworm (Arenicoia) - a study in adaptation. Neth. J. Sea Res., Vol. 3, pp. 294-3 13.

CLARK,