Hassan B. Akberali

Behavioural responses of the bivalve Scrobicularia plana (da Costa) subjected to short-term copper (Cu II) concentrations

Abstract Short-term (6 h) behavioural responses and mortality of Scrobicularia exposed to low concentrations of copper in sea water (31%, S) have been investigated. Scrobicularia can detect copper at a concentration of 0·01 ppm. The initial response is valve closure resulting in a rapid drop in heart rate. In concentrations of 0·05, 0·01 and, to a lesser extent, 0·1 ppm copper added to sea water, the clams begin to interact with the polluted water after 2–3 h. In 0·5 ppm, the valves remain closed and the heart rate is maintained at a low level over the 6-h exposure period. Mortality increases with time in 0·5 ppm copper concentration, reaching 50% in 5–7 days. In 0·05 and 0·01 ppm, no mortality was recorded over this period. There is an increase in mantle cavity water calcium concentration after 6 h in 0·5 ppm, in contrast to 0·1, 0·05 and 0·01 ppm. Aspects of anaerobiosis, accumulation of heavy metals and survival are discussed in relation to the behaviour of Scrobicularia exposed to various copper concentrations.

Behaviour of Scrobicularia plana (Da Costa) in water of various salinities

Abstract Valve movements, heart and pumping rates have been recorded from Scrobicularia plana (Da Costa) when transferred stepwise from natural sea water to sea water of S = 30% (termed 100%) and then to 80, 60, 40, and 20% sea water, and after direct transfer from 100 to 20% Scrobicularia exhibits short periods of pumping alternating with short ventilatory pauses down to 40% sea water. These are termed activity cycles and contrast with periods of complete quiescence each ≈ 4–12 h long. The heart rate of animals in these salinities is fairly constant during activity cycles. When transferred to 20% sea water, either directly or gradually, short pumping phases alternate with long ventilatory pauses during the initial 72 h. At the same time the heart rate shows wide variation during activity cycles. On transference from 100 to 60% sea water, the valves opened and valve activity pattern returned to normal within 30 min and within 6 h of transfer from 60 into 40% SW. Transfer to 20%, whether stepwise or directly, resulted in valves remaining closed initially for ≈ 10 or 34 h, respectively before opening gradually to expose the mantle margin. The valve adductions were then of reduced frequency and extent.

The action of heavy metals on the gametes of the marine mussel, Mytilus edulis (L.)—III. The effect of applied copper and zinc on sperm motilityin relation to ultrastructural damage and intracellular metal localisation

Treatment of Mytilus edulis sperm with external concentrations of copper or zinc (0-1-3-3mM) causes a decrease in motility in which zinc is more inhibitory than copper. Zinc also appeared to cause more extensive mitochondrial damage, as revealed by transmission electron microscopy, than did treatment with copper. The relationship between sperm motility and respiration in the presence of the various heavy metal concentrations used indicates that the depression of sperm motility can be explained largely on the basis of respiratory inhibition. However, zinc produces a less pronounced effect on sperm motility than on r respiration. X-ray microanalysis of thick sections of fixed treated sperm showed that copper accumulation occurs in the acrosomes, mitochondria and nuclei, whereas zinc is found in the acrosomes and in mitochondrial granules in association with calcium and phosphorus. No evidence was obtained for zinc accumulation in the nuclei. Treatment with either copper or zinc resulted in considerable reductions of bound calcium and phosphorus in both the acrosomes and mitochondria. It is suggested that the heavy metal ions cause an increase in the permeability of the organelle membranes to calcium and phosphorus. It is concluded that the less marked effect of zinc on sperm motility as compared to respiration may be due to an increase in the cytosolic free calcium concentration which, in turn, may stimulate the flagellar contractile apparatus.

Behavioural and siphonal tissue responses of Scrobicularia plana (bivalvia) to zinc

Abstract The effects of zinc being added to sea water, to final concentrations of 0·1–20 ppm, 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 10 ppm at 10°C. The median lethal times for 10 and 20 ppm zinc were 143·5 and 116·1 h respectively. The responses of Scrobicularia to zinc concentrations in sea water of between 0·1 and 10 ppm were tested by exposure for 6 h. Above 1 ppm, valve closure and bradycardia occur within 10–15 min. Below 5 ppm the valves subsequently opened and heart rate increased over the 6 h period, but in 10 ppm closure and pronounced bradycardia were maintained throughout. Addition of zinc, to final concentrations of 1, 5 and 10 ppm, 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 (1–10 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 of an isolated siphon.

45Calcium uptake and dissolution in the shell of Scrobicularia plana (da Costa)

Abstract 45 Ca incorporation in the shell of the bivalve Scrobicularia plana (da Costa) has been investigated using autoradiographic and counting techniques. The outside (periostracal side) of the valve incorporates about 25°, of the total 45 Ca deposited in the shell, possibly by passibly passive exhange or adsorption. The deposition of 45 Ca by unfed Scrobicularia , in which the valves had been painted with varnish, has been estimated to be 0.228 ng 45 Ca per valve for the 72-h experimental period. The location of calcium secretion on the inner surface of the valves of Scrobicularia has been determined and loss of 45 Ca from the shell during anaerobiosis has been demonstrated.

The action of heavy metals on the gametes of the marine mussel, Mytilus edulis (L.)-II. Uptake of copper and zinc and their effect on respiration in the sperm and unfertilized egg

The addition of stock copper and zinc nitrate solutions to filtered seawater (pH 7·91) resulted in a large pH shift which was more pronounced with copper. The pH shift was minimized by increasing the buffering capacity of seawater using 10 mm Tris-Tes pH 7·91. Subsequent experimentation was carried out in both unbuffered and buffered seawater. In unbuffered seawater, both copper and zinc had a pronounced inhibitory effect on Mytilus edulis sperm respiration with 50% inhibition occurring at 0.65 mm (41·3 ppm) copper and 1.0 mm (65·4 ppm) zinc. Egg respiration was also inhibited by copper with 50% inhibition occurring at 0.9 mm (57·2 ppm) whereas zinc inhibited egg respiration by only 30% at 1·5 mM (98·1 ppm). In buffered seawater, both copper and zinc produced an inhibitory effect on sperm respiration. However, with copper the inhibition was much reduced whereas zinc had tthe same inhibitory effect as in unbuffered seawater. In contrast to unbuffered seawater, egg respiration was stimulated by copper but zinc produced a comparable respiratory inhibition. The uptake of both copper and zinc in sperm and egg in buffered seawater increased with increasing metal concentration. Metal ion uptake, when expressed as ng ions μl cell volume−1, was approximately 3-fold greater in the sperm than in the eggs and in both cases zinc uptake exceeded copper uptake. At a seawater pH of < 7·5, in the absence of copper or zinc, the respiration of both egg and sperm was inhibited. The maximum levels of inhibition at pH 5·4 were 15% and 30% for sperm and egg, respectively. The differing actions of copper on egg respiration in buffered and unbuffered seawater appear to be due to enhanced copper uptake in unbuffered seawater.

Studies of the effects of zinc on the respiration of mitochondria from different tissues in the bivalve mollusc Mytilus edulis (L.)

Effects of Zn2+ concentrations have been studied on the respiration of mitochondria isolated from the mantle tissue and digestive gland of Mytilus edulis, and compared with rat liver mitochondria. 2. Both types of isolated M. edulis mitochondria had respiratory control indices (RCI) with ADP of 1.5–2.0 compared with 5–6 for rat liver mitochondria. 3. Approximate Zn2+ concentrations giving a 50% inhibition of succinate plus glutamate oxidation are 70 μM for rat liver mitochondria, 135 μM for M. edulis mantle mitochondria and 280 μM for digestive gland mitochondria, 4. These differences are statistically significant at P < 0.001 to P < 0.05.

The action of heavy metals on the gametes of the marine mussel, Mytilus edulis (L.)—I. copper-induced uncoupling of respiration in the unfertilized egg

The direct addition of Cu2+ to unfertilized eggs of Mytilus edulis results in a stimulation of respiration with maximal stimulation occurring at a Cu2+ concentration of ca 0.5 mM. By contrast, the addition of Zn2+ has no effect on egg respiration. The uncoupler CCCP produces a 5/6 fold stimulation of egg respiration but the addition of ADP leads to only a small release of respiration. In contrast, sperm respiration is unaffected by Cu2+, inhibited by Zn2+ and CCCP produces only a small respiratory stimulation. The addition of Cu2+ to respiring Mytilus mantle tissue mitochondria produces an initial stimulation of State 4 oxidation which is then followed by a progressive inhibition. It is suggested that respiration in the unfertilized egg may be inhibited by a high ATP/ADP ratio in the cytosol. Respiration can, therefore, be released by either the addition of a H+-translocating uncoupler or by Cu2+ which may act by stimulating mitochondrial K+ influx.

The detection of salinity changes by the marine bivalve molluscs Scrobicularia plana (da Costa) and Mytilus edulis L.

The bivalve molluscs Scrobicularia plana (da Costa) and Mytilus edulis L. both respond to falling external salinities by detecting changes in the concentration of a group of ions (rather than to osmotic pressure changes or changes in the concentration of a single ion). Scrobicularia is sensitive to Na+, Mg2+, Ca2+ (and possibly Cl−). The salinity sensing structures are deeply situated in S. plana (in contrast to the peripheral location in Mytilus); isolated siphons exhibit no reaction to salinity. Ablation of the visceral ganglion completely abolishes the siphon retraction response to low salinities whereas removal of the cruciform muscle complex, foot or gills does not. It is suggested that the vulnerability of Scrobicularia to periodic siphon loss (largely because of predation) is the reason for such siting.

The responses of the bivalve Scrobicularia plana (da Costa) to gradual salinity changes

Abstract The effects of gradual changes in salinity on the behaviour of normal Scrobicularia plana (da Costa) and on clams with the cruciform muscle complex surgically interfered with have been studied. The salinity of the retained mantle cavity fluid and the salinities associated with various components of siphonal activity in normal Scrobicularia are influenced by the rate of external salinity change. The responses of “operated” Scrobicularia to a decline in salinity occur at significantly lower salinities than in normal animals. It is suggested that the cruciform muscle complex in Scrobicularia is partly involved in sensing salinity changes in the external environment. The pattern of shell valve activity differs strikingly between normal and “operated” Scrobicularia exposed to a decline in salinity. At external salinities between 15 and 0‰, progressive valve closure in normal Scrobicularia is followed by slight valve movements. In “operated” Scrobicularia valve closure is, however, followed by intense valve activity. With dye uptake experiments, it has been shown that valve closure in Scrobicularia effectively isolates and protects the tissues from sudden changes in external salinity.