behavioural and siphonal tissue responses of scrobicularia plana (bivalvia) to zinc

Marine Ent, ironmental Research 5 (1981 ) 251-264

BEHAVIOURAL AND SIPHONAL TISSUE RESPONSES OF SCROBICULARIA PLANA (BIVALVIA) TO ZINC

H. B. AKBERALI, T. M. WOSG* and E. R. TRUEMAN

Department of Zoology, University o f Manchester, Oxford Road, Manchester M13 9PL, Great Britain

(Received: 3 October, 1980)

ABSTRACT

The effects of zinc being added to sea water, to final concentrations of O.l-2Oppm, have been studied on the heart rate, valve movements, mortality of Scrobicularia and on both isolated and in situ inhalant siphon preparations.

The acute toxicity threshold for added zinc was determined to be about lO ppm at IO°C. The median lethal times for 10 and 20ppm zinc were 143"5 and l16.1h respectively.

The responses of Scrobicularia to zinc concentrations in sea water of between O" 1 and lOppm were tested by exposure for 6h. Above lppm, valve closure and bradycardia occur within 10-15 min. Below 5 ppm the valves subsequently opened and heart rate increased over the 6h period, but in lOppm closure and pronounced bradycardia were maintained throughout.

Addition of zinc, to final concentrations of 1, 5 and lOppm, had no effect on the isolated inhalant siphon in contrast to copper at 0"25 and 0"5 ppm which caused very marked siphonal contractions. However, when zinc ( l-lO ppm) was added to an in situ inhalant siphon preparation, contractions occurred which were lost on removal of zinc from the bathing medium. Cutting the cruciform muscles medially resulted in the loss or delay of the response to zinc. This suggests the possibility of the cruciform muscle complex (muscle strands and associated sense organs) responding to zinc. This contrasts with the direct action of copper on the neuromuscular system o fan isolated siphon.

INTRODUCTION

Zinc is one of the more abundant toxic heavy metals found in estuaries and coastal waters, originating from industrial discharges as well as from mining operations

* Present address: School of Biological Sciences, University of Science Malaysia, Penang, Malaysia.

251 Marine Environ. Res. 0141-1136/81/0005-025 i/$02.75 © Applied Science Publishers Ltd, England, 1981 Printed in Great Britain

252 H . B . AKBERAL1, T. M. WONG, E. R. TRUEMAN

(Halstead, 1972). In recent years a considerable body of information has been accumulated on various levels of zinc, in solution and in deposited sediments in coastal waters, and on its bioaccumulation in various bivalve species (Benon et al., 1978; Abdullah et al., 1972; Eisler, 1977; Eisler & Henneky, 1977; Coombs, 1972; Bryan & Uysal, 1978; Bryan & Hummerstone, 1978; Bryan, 1979). There is, however, little known about the behavioural response of estuarine bivalves to sublethal levels of zinc or any metal ions.

Scrobicularia plana (da Costa), a bivalve commonly living in estuarine mud (Green, 1957), is primarily a deposit feeder but switches to suspension feeding when constantly submerged (Hughes, 1969; Earll, 1975). Scrobicularia has been shown to accumulate metals (Bryan & Hummerstone, 1977; Bryan & Uysal, 1978) and as such has been proposed as a useful indicator organism for heavy metal pollution (Bryan & Hummerstone, 1978).

Akberali & Black (1980) have recently demonstrated that Scrobicularia reacts to copper at levels of between 0.01-0.5 ppm and responds by valve closure thereby isolating its tissues from the polluted environment. Similar isolating responses to low levels of added copper have also been reported for Mytit,~s edulis (Davenport, 1977; Davenport & Manley, 1978) as well as other bivalves (Manley & Davenport, 1979). As the existence of such isolating response to sublethal levels of pollution can affect the animal's reliability as an indicator of periodic high levels of pollutant discharges, it is of interest to determine whether a similar isolating mechanism exists for other heavy metals in Scrobicularia.

Scrobicularia possesses extensible inhalant and exhalant siphons through which the clam, usually buried to a depth of 5-20 cm, makes contact with the overlying sea water. The inhalant siphon of Scrobicularia comprises of longitudinal, circular and radial muscles with six longitudinal nerves (Yonge, 1949; Chapman & Newell, 1956). The posterior pallial nerves pass from the visceral ganglion to the siphonal nerves and innervate the longitudinal muscle fibres. The longitudinal muscles, which are the major muscular component, cause the siphons to retract rapidly into the shell. This occurs as the first visible response of Scrobicularia to adverse environmental stress conditions. Recently an isolated inhalant siphon preparation of Scrobicularia has been developed and its response to various copper concentrations studied (Akberali, unpublished).

This paper reports the results of studies aimed at determining whether an isolating response exists for Scrobicularia with respect to zinc, and if so, the threshold levels at which such responses are triggered off. The response to soluble zinc was studied since it is not feasible to keep levels of zinc in sediments under rigorous control levels. The effect of zinc solutions at various concentrations on inhalant siphonal preparations of Scrobicularia have also been studied in relation to the behaviour of the intact animals. This investigation also provides an opportunity to compare the effects on Scrobicularia of two heavy metals, zinc and copper.

ZINC EFFECTS ON Scrobicularia 253

MATERIALS AND METHODS

Scrobicularia plana, 4-4.5 cm overall shell length, were collected from Morecambe (Lancashire) at mid-tide level and kept in mud covered with well aerated sea water (31%o, S) at l0 °C, for about one week before the start of any experimentation. All clams were used within three to four weeks.

Zinc was added as Zn(NOa) v Stock solutions of 100 mg/ml ( = ppm) and 10 ppm zinc in sea water were prepared from 1000 ppm atomic absorption standard solution (BDH) and their pH brought up to 7 with KOH. In the process part of the zinc precipitated out of solution. After filtration the zinc concentration in the stock solutions as well as that in the final test solutions in sea water were checked against standards using a Pye Unican SP 90 A atomic absorption spectrophotometer. The following final zinc concentrations were used in normal sea water: 0. l, 0-5, l, 5, l0 and 20 ppm. Preliminary experiments showed that the pH ( > 6) of the sea water had no effect on the activity of the animal. In order to minimise this further, the pH of final test zinc solutions in sea water were adjusted to between 7.2-7.6.

Appropriate volumes of stock zinc and copper solutions were added to 150 ml sea water in the organ bath, giving final concentrations of l, 5 and l0 ppm for zinc and 0.25 and 0.5 ppm for copper, in order to study their effect on an inhalant siphon of Scrobicularia. Rapid mixing was achieved by continuously bubbling air through a capillary tube in the organ bath.

Survival of Scrobicularia in sea water with added zinc Scrobicularia normally occurs buried in mud, but in contact with overlying sea

water through its siphons. This arrangement was not suitable for use in toxicity experiments which were carried out using a continuous exposure procedure. The clams were placed in plastic tanks each containing 5 litre of well aerated sea water to which zinc stock solution was added to give the desired concentration. After pilot experiments the following final zinc concentrations were selected: 0.1, 0.5, 1, 5, 10 and 20 ppm. For each of these, ten animals were individually marked and placed in tanks containing sea water with added zinc. Animals were inspected continuously during the first hour following transfer and twice daily thereafter. Animals were considered dead if they gaped and showed no mantle, siphonal or pedal movement in response to tactile stimulation. At each inspection, the extension of siphons, foot and extent of valve opening was also noted. The sea water with added zinc was changed every 2 days. Toxicity tests were continued for at least 8 days, since Scrobicularia is able to effectively isolate itself from adverse environmental conditions for periods of 5-7 days (Akberali, 1978), rather than for 48 or 96h as commonly used on other marine invertebrates (Ahsanullah, 1976; D'Silva & Kureishy, 1978; Reish & Carr, 1978).

254 H. B. AKBERALI, T. M. WONG, E. R. TRUEMAN

Heart rate and valve movements o f Scrobicularia in sea water with added zinc The behavioural response of Scrobieularia was monitored on a George

Washington Oscillograph Model 400 MD/2 by recording the movements of the valves using isotonic transducers (C. F. Palver) and the heart rate using the impedance pneumograph technique (Trueman et al., 1973; Akberali, 1978). After the initial preparation for recording, animals were left to recover for 24 h in 1 litre of sea water aerated with stone diffusers. The sea water was changed 2 h before the addition of zinc, and after this, recordings commenced. Zinc was added from prepared stock solutions by means of a ,burettel at the rate of approximately 5-7 ml/min, with the nozzle directed against the air diffusers to permit sufficient mixing before coming into direct contact with the animal. Preliminary tests using sea water showed that this dosing procedure without zinc did not have any effect on the activity of animals. Samples of the test media were taken for the determination ofpH and zinc concentration about 30 min after dosing and at the end of exposure period. A 6 h exposure period was chosen for these experiments to reflect the approximate duration of normal tidal regimes and there was little change in either the pH or level of added zinc during this period.

After 6 h, the test medium was replaced with 1 litre fresh sea water previously cooled to 10°C. Preliminary tests showed that the emptying, rinsing and filling procedure employed resulted in immediate siphonal retraction and valve closure in active animals. However, little change occurred in the heart rate and in all cases, opening of the valves was followed by siphonal extension within a few minutes. The only exception to this was when one test medium of high zinc concentration was replaced by another of the same zinc concentration. Heart rates were analysed by counting the number of beats over 5 min segments and presented as the mean beats/min at 10 min intervals.

Effects o f added zinc on isolated and in situ inhalant siphon preparation of Scrobicularia

The shell was opened by cutting the adductor muscles very close to their attachment to the right valve and both the inhalant and exhalant siphons isolated by removing the siphonal retractor muscles and cruciform muscle and releasing the siphons from the mantle. The whole procedure was carried out in sea water. The isolated inhalant siphon was fixed, by a pin through the ligatured base, in an organ bath containing 150 ml normal sea water.

In situ studies were carried out on the inhalant siphon of Scrobicularia after removing the right valve and carefully leaving the underlying mantle lobe intact. The clam, now intact on a single valve, was left covered in sea water for I0--15 min when both siphons extended. The inhalant siphon was then ligatured quickly with thread near its tip and the valve fixed to the base of an organ bath with black wax.

The ligatured tip of both the isolated and in situ siphonal preparation was attached to the lever of an isotonic transducer which was connected to a pen

ZINC EFFECTS ON Scrobicularia 2 5 5

recorder. The tension on the thread was adjusted very gradually (to about 5 x 10-3N) to extend the isolated and in situ inhalant siphonal preparation to standard lengths of 3 and 2 cm respectively, measured from the base of the siphon to the ligatured inhalant tip. This length represents the base line of the ligatured siphonal preparations in the relaxed state in the recordings.

RESULTS

Survival o f Scrobicularia in sea water with added zinc The mortality of Scrobicularia when continuously exposed to sea water with

various levels of added zinc is shown in Fig. 1. Over the 8 days experimental period

100-

A

5(>

0

~ °°.. .4 / /

,/

, ' / O J

4 6 Days

Fig. 1. Mortality of Scrobicularia at 10°C when exposed to various levels of added zinc over 8 days. Solid line indicates survival of all animals at all zinc concentrations over the first 3 days. A - - A = I ppm; A - - - A = 5 ppm; O - - O = 10ppm; I - - - I = 20ppm. Points shown are means o f 2 separate

experiments (N = 10 for each). Vertical bars indicate range of variation in results.

no mortality occurred in the control as well as those exposed to 0.1,0.5 and 1 ppm added zinc (all survived for 14 days). At 0.1 and 0.5 ppm added zinc, the animals interacted freely with the environment, with the opening of valves and extension of siphons within 15-30 min of transfer but only a few animals in I ppm zinc showed similar behaviour over the first hour. At higher zinc concentrations, few animals opened their valves during the first day and in 20 ppm zinc no siphonal extensions were recorded in any of the inspections carried out over the first 3 days. In both 10 and 20ppm zinc, numerous siphonal segments were found broken off. Similar siphon breakages were observed when Scrobicularia were exposed to copper or subjected to low salinity stress. It appears that the breaking of siphon segments was caused by the rapid and vigorous closure of valves in response to the adverse environment. In 5, 10 and 20ppm zinc, the first mortalities occurred between 84--96h and the total mean mortality over 8 days was 25%, 65 % and 100~o


respectively. The mean lethal time for 50 ~o mortality (M LT) in 10 and 20 ppm zinc based on probit analysis were 143-5 and 116-1 h respectively. Based on these results 10 ppm added zinc appears to be the acute zinc toxicity threshold in Scrobicularia from Morecambe Bay.

Heart rate and valve movements o f Scrobicularia in sea water with added zinc

Typical recordings of valve movements and heart rate for the first 30 min after addition of various levels of zinc are presented (Fig. 2). At zinc concentrations of between 0.1-0.5 ppm the clams do not behave differently from the controls. At higher concentrations however, the siphons were withdrawn and valves closed with a marked lowering of heart rate but in most animals this was followed within 20 min by a series of valve adductions (Fig. 2). In some Scrobicularia in 1 ppm zinc this leads to a partial resumption of activities. At zinc concentrations higher than 1 ppm such adductions were however, followed by prolonged valve closure although further periodic valve adductions were evident over the 6 h period. In animals exposed to 10ppm zinc, sustained valve closure occurred over the experimental period accompanied by pronounced bradycardia (Fig. 3). At lower zinc concentrations the

-5Dpm Zn

~ n •

A~ ltDOm Zn

dum~l.,lami , = , , ~ , j ~ --t t . . . . ,~ - ~ + - - - * ~ i # * ~ , ~

¢1~

_ z o

10 I~1~ Zn

• 14. oo

0 l0 20 30 MINUTES

Fig. 2. ExamplesofrecordingsofheartbeatandvalvemovementsofScrobiculariaat 10=Conaddition of various concentrations of zinc. A(,t) indicates addition of stock zinc solution, B(J,) indicates attainment

of final zinc concentrations, op = open, cl = closed.


2O j, , ,.. " iv - - , . , ; .

~-16 • "d

~10- 1 °

4 . . . . ~>. , ~ , ,~,, , ~ . . ~ , . ~ , .~ . . . . .

Hours

Fig. 3. Effects of various levels of added zinc in normal sea water on the heart rate of Scrobicularia at 10°C. Arrow 0,) at Oh indicates application of test concentration of zinc, while (1") at 6h indicates replacement with fresh sea water. Each point is a mean of 4-8 animals. Vertical bars represent the standard error. I - - I = 0.5 ppm zinc (N = 4); i - - A = 1 ppm zinc (N = 7); A - - - A = 5 ppm zinc (N = 5); • - - • = I 0 ppm zinc (N = 8). 50 ml normal sea water ($) applied to the controls (O---C),

N = 5). Treatment for controls after 6 h as in test situations.

frequency and duration of interaction with environment became greater over the 6 h period. This is reflected by increase in heart rate (Fig. 3).

Replacement of the test solutions with clean sea water resulted in a rapid resumption of activity (Fig. 4(B)). An initial series of testing valve adductions is evident within 5-10min, followed by the progressive opening of the valves and

A ~ ~d

MINUTES

Fig. 4. Examples of recordings of heart beat and valve movements of Scrobicu/aria at 10 °C in control (A) and animal exposed to 10 ppm added zinc (B) when test solutions were replaced by fresh sea water after 6h; c(D-d(D and e(D-d(D indicate duration of removal of test solutions and subsequent

replacement with fresh sea water.


extension of siphons. Accompanying this is a marked increase in heart rate, often to beyond normal active values (Fig. 3 and Fig. 4(B)). This overshoot response is most marked in animals subjected to 5-10ppm zinc. In contrast such hyperactive responses were not evident in either the control animals or those exposed to zinc concentrations of less than 0.5 ppm (Fig. 3 and Fig. 4(A)), indicating that the isolating response observed appears to be due to the presence of zinc at concentrations above 1 ppm.

Effects of added :inc on siphonal preparations of Scorbicularia The application of 1,5 and 10 ppm zinc directly to the isolated inhalant siphon of

Scrobicularia has no apparent effect (Fig. 5). After the removal of 10 ppm zinc in sea water from the organ bath (Fig. 5(C)) followed by washes, the isolated siphonal preparation was left in fresh sea water (Fig. 5(D)) for 10 min. The siphon was then

~Ippm Zn J,~Zn ,l.lOm~orn Zn c¢---w ,; o ,;.25=mcu ,; 5~c~

0 10 20 30 40 50 60 70 80

Minutes

Fig. 5. Effects of various zinc and copper concentrations on an isolated inhalant siphon preparation of Scrobicu/aria. Arrows (~) mark the application of zinc and copper stock solutions to give the indicated concentrations. Upward deflection of the trace indicates isotonic contraction of the inhalant siphon. In sequence the following events took place: Removal of 10 ppm zinc sea water (C), followed by three separate washes (W); replacement of sea water (D); contraction, usually associated with the removal of the bathing medium in the period C-D; mechanical stimulation (S). This recording is a typical example of

six preparations.

tested with mechanical stimulation (e.g. gently squeezing with forceps) and responded by contracting (Fig. 5(S)). Following this period in fresh sea water, application of copper to give a final concentration of 0.25 ppm in the organ bath (Fig. 5) resulted in siphonal contraction after 7-8 min. Further application of copper to a final concentration of 0-5 ppm (Fig. 5) causes the siphon to respond with a series of contractions after 2-3 min and the siphons remained in a prolonged contracted state in the presence of 0.5 ppm copper. These responses to copper are similar to those observed for this isolated siphonal preparation (Akberali, unpublished). They indicate that at the concentrations used, the isolated siphon is neither reacting to nor is it apparently being affected by solutions of zinc salts, whereas copper at low concentrations has a marked effect.

When the same concentrations of zinc were added to an in situ preparation, a marked response was obtained from the inhalant siphon (Fig. 6). Application of 1 ppm zinc, resulted in contractions of the siphon but these were not uniform in respect of their timing or number (Figs 6 and 8). The response was enhanced at


5 ppm and further application of zinc to give a final concentration of 10 ppm in the organ bath resulted in the siphon showing a series of spontaneous contractions before remaining in a contracted state with the siphon withdrawn to the valve margin (Fig. 6). Removal of the sea water containing zinc at 10ppm (Fig. 6(C)), followed by washes and replacement with fresh sea water (Fig. 6(D)) resulted in the loss of siphonal contractions. This siphonal response of an in situ preparation is thus likely to be caused by the presence of zinc in the bathing medium. Application of mechanical stimulus during the recovery period restilts in the siphon responding by contraction (Fig. 6(S)).

$1p~nZn ~, 5pl~n Zn ~I 10pl3m Zn C~ w"~-~D

Fig. 6.

0 10 20 30 40 50 Minutes

Effects of zinc on an in situ inhalant siphon preparation of Scrobicu/aria. (Letters and procedure as in Fig. 5.) This recording is a typical example of six preparations.

Previous work (Odiete, 1978) indicated the possibility that the cruciform muscle complex in Scrobicularia has a chemoreceptive function. The complex consists of paired muscle strands passing between the valves in the form of a cross and a pair of sense organs with associated ganglia (Fig. 7). It is located between the inner lobes of the mantle margin close to the base of the inhalant siphon (Graham, 1934; Yonge, 1949; Moueza & Frenkiel, 1974; Odiete, 1978). The comPlex appears to have a dual function, facilitating siphonai movement and acting as a chemoreceptor. Odiete (1978) detected impulses from the sense organ ganglia in the presence of'foul water' and demonstrated that tension in the cruciform muscle tends to dilate the intramuscular slits, causing water to be sucked in over a sensory epithelium. The intramuscular slit collapses when tension is removed.

In this present investigation we made a longitudinal median incision through the muscles (Fig. 7) so as not to affect the pallial nervous system directly. However, the cut reduces tension in the cruciform muscles and in consequence the mechanism for drawing water over the sensory epithelium must be less effective. Preliminary experiments were carried out to investigate whether the response (Fig. 6) of the in situ siphonal preparation of Scrobicularia involves the detection of zinc in the medium by the cruciform muscle complex. The in situ preparation was first subjected to 1 and 5 ppm zinc (Fig. 8(A,B)) with responses which were similar to those prexiously observed (Fig. 6). However, when the cruciform muscle complex was cut by a longitudinal median incision (Fig. 7 and Fig. 8(X)) the response to 10 ppm zinc was

260 lq. B. AKBERALI, T. M. WONG, E. R. TRUEMAN

SLIT

VALVE

Fig. 7. Diagrammatic representation of a horizontal longitudinal section (posterior aspect above) of the posterior valve margins and cruciform muscle complex of Scrobicularia. Broken line indicates the plane of the median incision referred to in the test. Sensory organs consist of a deep pouch opening to the exterior by a papilla and terminating in an intra-muscular slit (slit). Sensory cells are located in the

pouches adjacent to the ganglia (after Odiete 1978, and our observations).

,[ 'lppm Zn X,,W,10ppm Zn ~ _ ~ . ~ n S~._~

0 10 20 30 40 B

~,lppm Zn , l , 5 ~ X ~ l O ~ . m Z n

0 10 20 30 Minutes

Fig. 8. Effects of various zinc concentrations on in situ inhalant siphon preparations of Scrobicularia. Arrows (~) mark the application of zinc stock solution to give the indicated concentrations. X--cutting of the cruciform muscle complex before addition of 10 ppm zinc. A, B---Two examples of the recordings observed. S--Mechanical stimulation. These recordings are typical examples from six preparations.


lost, although the siphon remained sensitive to direct mechanical stimulation (Fig. 8(A,S)). In a few instances (Fig. 8(B)), a weak and delayed response was recorded in the presence of 10ppm zinc.

DISCUSSION

The results of this study indicate that Scrobicularia is able to tolerate exposure to relatively high levels of added zinc in sea water for up to 14 days. At 10°C, the acute toxicity threshold for added zinc is approximately 10 ppm. However, behavioural experiments have demonstrated clearly that Scrobicularia responds to the presence of zinc in solution at much lower concentrations, with threshold levels between 0.5- 1 ppm. The clam responds to this by siphonal withdrawal and valve closure, thus effectively isolating itself from the polluted environment. Associated with this isolation response is a pronounced bradycardia suggesting a more general depression of metabolic activities as is evident from similar studies on Scrobicularia and other bivalve molluscs (Helm & Trueman, 1967; Brand & Roberts, 1973; Earll, 1975; Brand, 1976; Akberali, 1978).

Valve closure responses were evident initially in 1 ppm added zinc but the test animals subsequently interacted fully with the environment, the long term toxicity test showing that 100 ~ survival was possible at this concentration for up to 14 days. Zinc concentrations above 1 ppm result in increased periods of isolation from the environment, this being most marked at 10 ppm added zinc. Such closure responses by Scrobicularia will undoubtedly protect the animal from the damaging effects of short term exposure to even fairly high concentrations of zinc. It has been previously observed (Akberali et al., 1977; Akberali & Trueman, 1979), that when Scrobicularia closes its valves in response to stress, the animal nevertheless maintains some contact with the environment by slight exposure of the mantle edge. This would allow the clam to detect changes in the environment and respond rapidly when environmental conditions become more favourable. Akberali et al. (1977) and Akberali & Black (1980) have shown that during such periods of valve closure the clams respire anaerobically in order to maintain basal metabolism. Obviously there is a limit to duration of anaerobic respiration and the animal must eventually eliminate its accumulated wastes. When such enforced opening of the valves occurs, the tissues come into contact with the pollutant which may cause eventual death.

The level of added zinc at which more than 50 ~ mortality occurs in Scrobicularia is of the same order of magnitude recorded for other adult bivalves such as Mya arenaria (Eisler, 1977), Mytilus viridis (D'Silva & Kureishy, 1978), M. edulis planulatus and Neotrigonia margaritacea (Ahsanullah, 1976) though LC 50's for Mercenaria mercenaria and Crassostrea virginica were found to be between 0.1- 0.3 ppm zinc (Calabrese & Nelson, 1973; Calabrese et al., 1973). The sensitivity of Scrobicularia to zinc, both in terms of the threshold for behavioural response as well as for acute toxicity is one order of magnitude lower than the corresponding values


for copper (Akberali & Black, 1980). A low threshold for the valve closure response to added copper has also been reported in M. edulis (Davenport & Manley, 1978) and in other bivalve molluscs (Manley & Davenport, 1979). Similar differences in the relative toxic effects of copper and zinc were noted in M. virMis (D'Silva & Kureishy, 1978).

An explanation of the differing sensitivity to copper and zinc may be suggested in terms of the direct effects of these two heavy metals on the inhalant siphon of Scrobicularia. Addition of zinc up to a final concentration of 10ppm has no apparent effect on the isolated siphon, whereas low concentrations of copper affect contraction. Related studies on the effects of copper have indicated that copper probably causes contractions by acting at the neuromuscular junction by indirectly affecting the release of transmitter acetylcholine (Akberali & Trueman, unpublished). The response of the in situ inhalant siphon preparation to zinc is probably sensory in nature and corresponds to the withdrawal of the siphons and associated valve closure observed in the whole animal, in response to high zinc concentrations.

The effects ofadded zinc only on the in situ siphonal preparation together with the loss of response when the cruciform muscles are cut (Figs 7 and 8) indicate the possibility of zinc affecting the sensory epithelia of the cruciform muscle complex. Delayed or weak responses to 10 ppm zinc occur in some preparations (Fig. 8(B)) after the cutting of the cruciform muscles. This could either be due to the ability of other parts of the mantle lobes to react to zinc or to the continued, but less effective, functioning of the cruciform sense organs. Increase in tension of the cruciform muscles dilates the intramuscular slits and draws water from near the base of the siphons over a sensory epithelium. Cutting the cruciform muscle complex may reduce the flow of water over the sensory eipthelium and diminishes the response to pollutants. It is likely that the tension in the cruciform muscles may be controlled independently of valve gape and so allow Scrobicularia to sample water entering the mantle cavity either when the valves are gaping or apparently closed (Akberali & Trueman, 1979). Further investigations of the cruciform muscle complex and of the innervation of the siphon are proceeding.

Davenport (1977) has suggested that the usefulness of M. edulis as a biological monitoring agent may be limited, since the mussel may fail to register transient or recurrent short-term presence of high pollutant level as a result of behavioural avoidance. This may equally apply to Scrobicularia with respect to copper (Akberali & Black, 1980). However, in the case of zinc, though Scrobicularia shows similar behavioural avoidance to high concentrations of soluble zinc, the level at which the valve closure response becomes operative is so high (5-10ppm) as to make it ineffective at the levels of zinc currently recorded in coastal waters (3-26 ppb-- Abdullah et al., 1972; Bryan & Uysal, 1978). At such concentrations Scrobicularia will remain active and interact with the medium so that any determination of zinc contained within the tissues will reflect the availability of zinc in the environment.


ACKNOWLEDGEMENTS

We would like to thank Dr J. H. Kennaugh for very kindly preparing sections of the cruciform muscles, Dr D. F. Cawthorne for his assistance with probit analysis and Mr L. Lockey for photography. This work was supported by an NERC Research Grant (GR3/3436).

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