Relationships between seston,available food and feeding activity in the common mussel Mytilus edulis

The feeding and metabolic rates of Mytilus edulis L. of different body sizes were measured in response to changes in particle concentrations ranging from 2 to 350 mg l^-1. Rates of oxygen consumption were not significantly affected by changes in seston concentration, whereas clearance rates gradually declined with increasing particle concentration. Pseudofaeces production was initiated at relatively low seston concentrations (<5 mg l^-1). Marked seasonal changes were recorded in the composition of suspended particulates (seston) in an estuary in south-west England. Total seston was sampled at frequent intervals throughout an annual cycle and analysed in terms of: particle size-frequency distributions, total dry weight (mg l^-1), inorganic content, chlorophyll a, carbohydrate, protein and lipid. The particulate carbohydrate, protein and lipid content provided an estimate of the food content of the seston. The results are discussed in terms of the food available to a nonselective suspension feeder, such as M. edulis, during a seasonal cycle. The effect of inorganic silt in suspension was mainly to limit by dilution the amount of food material ingested rather than to reduce the amount of material filtered by the mussel. In winter, the food content of the material ingested was 5%, and this increased to 25% during the spring and summer.     

Relationship between body zinc concentration and allometric growth measurements in the mussel Mytilus edulis

The frequency distribution of shell lengths in the mussels Mytilus edulis (L.) taken from a site in the Tyne Estuary (UK) in summer, 1980, typically displayed a bimodal shape, with a small peak in the 8 to 20 mm size range and a much broader peak in the 22 to 50 mm region. A collection of mussels was made consisting of two groups, one from each of the above size peaks. The group of larger mussels had a significantly higher mean zinc concentration than the group of smaller mussels, mainly because a few large individuals had very high zinc concentrations. The ratios of shell width:height, width:length and length:height were also significantly higher in larger mussels. However, the ratio of flesh dry wt:shell dry wt (flesh condition) was lower in larger individuals. The whole soft tissue zinc concentration was positively correlated with width:height and width:length, but negatively correlated with flesh wt:shell wt. It was concluded that allometric ratios may provide an attractive alternative to simple size characteristics as a basis for trace metal determinations, particularly in a comparison of metal levels in mussel populations from widely differing habitats where absolute size is a poor indicator of age or growth rate.Contribution No. 1221 of the Center for Estuarine and Environmental Studies of the University of Maryland     

Digestive rhythms in the mussel Mytilus edulis

Mytilus edulis L., collected from a mid-tide level on the shore, showed rhythmic changes in mantle fluid pH, crystalline style pH, style length and total protein, and in the amylase activity in the digestive gland. These changes were correlated with the changes in tidal height. Style size may be related to extracellular digestion in the stomach. Style size and amylase content of the style were not significantly correlated with each other. The changes in amylase activity in the digestive gland confirmed the existence of a tidal rhythm for intracellular digestion in M. edulis.     

Lead uptake from sea water and food,and lead loss in the common mussel Mytilus edulis

Common mussels, Mytilus edulis (shell length 19 to 21 mm, average dry weight 30 mg) were maintained for 6 weeks in sea water containing different concentrations of lead (0.005 to 5 mg · l^-1). The lead concentration in the mussels' whole soft parts was analysed at different times during the experiment. A constant rate of lead uptake, linearly dependent on the lead concentration of the medium, was observed. Thus, the temporal change of the concentration factor is also linear (regression coefficient 149.9 daily). Rate of lead loss, measured after transferring the mussels into natural sea water, is linearly dependent on the original lead concentration in the soft parts. Rates of uptake and loss in large mussels (shell length 45 to 55 mm, average dry weight 750 mg) are less than those in small mussels (shell length 19 to 21 mm, average dry weight 30 mg). During a much more extended experimental period, adjustment to a steady state is expected to occur; rates of lead uptake and loss are then non-linear. Lead uptake by individual organs (kidney, gills, adductor muscle, digestive gland, foot, mantle with gonads) of large M. edulis (shell length 45 to 55 mm, average dry weight 750 mg) was analysed in 2 test series. In the test series medium, the mussels were kept in a seawater medium containing 0.01 mg. Pb.l^-1. In the test series food, the mussels were kept in natural sea water but fed with the green algae Dunaliella marina containing lead (approximately 600 g.g^-1 dry weight). The lead quantity given per mussel per day was about 2 g in both test series. Within 35 days, the mussels of test series medium took up 29% of the total amount of lead given, those of test series food took up 23.5%. In all organs, lead concentration increased, but rates of uptake differed; the kidney displayed by far the highest rate of uptake. With these physiological properties M. edulis is an ideal indicator organism for lead pollution in the marine environment. A biologic calibration curve, the relationship between lead concentration in the mussels' whole soft parts at equilibrium and lead concentration in sea water, is presented.This paper forms part of a doctoral thesis in biology at Hamburg University     

Copper-induced differential mortality in the mussel Mytilus edulis

Copper toxicity experiments were performed with 3 000 mussels (Mytilus edulis L.) from a population that is a mixture of individuals originating in the Kattegat and in the Baltic. Subsequently, the electrophoretic patterns of the enzymes phosphoglucose isomerase (PGI) and phosphogluco mutase (PGM) were analysed in the dead and surviving mussels in order to detect possible differential mortality. Significant differences in allele frequencies between dead and surviving mussels were found for PGI. The results are discussed in relation to genotypic differences in the PGI and in relation to the composite structure of the investigated population. It is concluded that the differential mortality is due to genotypic differences in the PGI locus and/or to different susceptibility to copper by mussels originating in the two geographic areas.     

Relationship between cadmium concentrations in seawater and those in the mussel Mytilus edulis

Various forms of regression analysis are presented which show that there is a significant equilibrium relationship between total recoverable cadmium in seawater and its concentration in the mussel Mytilus edulis (P<0.001). The calculations show that the concentration of cadmium in seawater should not exceed 0.20 g l^-1 if the mussel is not to reach a cadmium concentration of 2 mg kg^-1 wet weight: a value frequently used as a food standard for human consumption. Further, when a cadmium value of 2 mg kg^-1 wet weight is reached, the concentration factor by mussels for cadmium from seawater is 9 950.     

The effect of copper and zinc on the metabolism of the mussel Mytilus edulis

The effects of copper and zinc on the metabolism of the mussel Mytilus edulis (L.) and its component tissues were studied. 500 ppm copper sodium citrate inhibited oxygen consumption of the whole animal and gill tissue, but no similar effect was observed on digestive gland tissue. 500 ppm zinc sodium citrate exerted no effect upon gill or digestive gland respiration, and neither metal salt affected the respiration of homogenates of gill, digestive gland or gonad. Direct observation of gill tissues during exposure to the metals revealed that 500 ppm copper sodium citrate caused inhibition of ciliary activity; exposure of tissues to 2 ppm Cu for 24 h resulted in only partial inhibition of the cilía. It is suggested that metabolic suppression noted in whole animals and gill tissues is due to the inhibition of an energy-consuming process such as ciliary activity rather than interference with respiratory enzyme systems.     

Hydrocarbon uptake and loss by the mussel Mytilus edulis

The dynamics of accumulation and elimination of hydrocarbons by the blue mussel Mytilus edulis were studied in a continuous-flow system. Mussels were exposed for as long as 41 days to 200 – 400 g/l of diesel fuel adsorbed on kaolin particles. Hydrocarbons were accumulated in the tissues in excess of 1000 times the exposure levels. Upon termination of dosing, the mussels exhibited a rather rapid loss of hydrocarbons for the first 15 to 20 days (biological half-life=2.7 to 3.5 days). Subsequently, however, elimination was reduced to a minimum and a considerable fraction of the hydrocarbons could be recovered from the tissues after as long as 32 days of depuration. The mussels exhibited definite signs of physiological stress due to chronic exposure to diesel fuel, although recovery was rapid upon termination of dosing. It is concluded that mussels could be utilized as a test organism for monitoring long-term hydrocarbon pollution in marine waters. The implications for the mussel culture industry are discussed.     

Accumulation of cadmium and bioenergetics in the mussel Mytilus edulis

Accumulation rates of cadmium, the amount of food ingested and assimilated, the amount of oxygen consumed and changes in dry flesh weight have been measured in Mytilus edulis L. exposed to 0, 10 and 100 ppb cadmium for 17 d in aquaria with seawater flowing continously and at constant algal concentration. The accumulation rates were linear at 10 and 100 ppb, amounting to 0.58 and 8.89 ppm d^-1, respectively. Body loads up to 150 ppm caused no effects on either clearance, ingestion, assimilation, respiration, or growth. High net growth efficiencies between 55–59% were obtained, indicating near optimal experimental conditions. It is suggested that the setup and experimental procedure provide an excellent tool in the study of accumulation and sublethal effects of environmental pollutants in suspension feeding bivalves.     

Ice nucleation in the blue mussel (Mytilus edulis)

The ice nucleation temperatures of different homogenised organs of the mussel Mytilus edulis L. were examined. The stomach and the hemolymph had the highest nucleation temperatures. In the homogenised stomach the nucleation temperatures were fairly constant throughout the year, whereas the nucleation temperatures of the hemolymph increased in the cold season. Bacterial growth experiments, microfiltration, and experiments using antibiotics indicated that the nucleators were not of bacterial origin. The nucleation temperatures of natural seawater were approximately −9 °C, whereas seawater made from distilled water and sea salt had nucleation temperatures of about −17 °C. The nucleation temperatures of the seawater were reduced when the seawater was filtered by the mussels. However, no clear indication that the nucleators in the stomach were obtained from the seawater was found. Stomach homogenates from mussels kept in nucleator-free water had the same supercooling points as stomach homogenates from mussels kept in natural seawater. This indicates that the nucleators in the stomach are not obtained from the seawater. The temperature and light conditions examined in the present study did not significantly influence the hemolymph ice nucleation temperatures of mussels kept in the laboratory. Received: 24 October 1996 / Accepted: 16 November 1996     

Relationship between lead concentrations in seawater and in the mussel Mytilus edulis: a water-quality criterion

Regression analysis on data collected from Port Phillip Bay and Western Port, Australia in 1979 shows that there is a significant equilibrium relationship between total recoverable lead in seawater and its concentrations in the mussel Mytilus edulis (P<0.001). The concentration of lead in seawater should not exceed 1.27 g l^-1 if the mussel is not to reach a lead concentration of 2.5 mg kg^-1 wet weight, a value frequently used as a food standard for human consumption. When a lead value of 2.5 mg kg^-1 wet weight is reached, the concentration factor by mussels for lead from seawater is 1969. The critical value of 1.27 g l^-1 could be used as a marine water-quality criterion for lead in waters where mussels are harvested.     

Oogenesis in the marine mussel Mytilus edulis: an ultrastructural study

Ultrastructural changes occurring during the course of development in oocytes of Mytilus edulis are described for mussels collected at monthly intervals over a period of one year (September 1981 to October 1982) from a site in Cornwall, England. During early stages of oogenesis the oocyte is surrounded by a small number of follicle cells but, as development proceeds, the follicle cells are restricted to the stalk region which attaches the oocyte to the acinar wall. Contact between the follicle cells and the developing oocyte is maintained by means of desmosomelike gap junctions. Organelles and inclusion bodies present in the ooplasm during oogenesis include rough endoplasmic reticulum (RER), Golgi bodies, mitochondria, free ribosomes, Balbiani's vitelline body, annulate lamellae and yolk and cortical granules. The RER, in particular, varies considerably throughout the course of development. Evidence for uptake of exogenous macromolecules into oocytes by pinocytosis is presented; it occurs in the basal region of previtellogenic oocytes prior to the formation of the vittelline coat. Lipid-yolk granules invariably have mitochondria in close association and, during the winter months, develop in close proximity to small, apparently glycogen-rich vesicles possibly suggesting that conversion of glycogen to lipid takes place in developing oocytes. Oocyte degeneration was commonly observed and involves initial breakdown of the plasma membrane followed by rupture of the vitelline coat. The oocyte contents once released into the acinar lumen are resorbed by the epithelial cells of the gonoducts, which are prevalent throughout the mantle of ripe individuals.     

Metal interaction during accumulation by the mussel Mytilus edulis planulatus

Mytilus edulis planulatus (Lamarck) were collected from Howden, South-east Tasmania in autumn 1981. Interaction effects of cadmium, copper and zinc during accumulation by mussels exposed for ten days to all three metals simultaneously were examined in a series of experiments in which each metal was tested at three concentrations. In general, interaction effects were most evident at the highest concentrations tested (20 g l^-1 Cd; 20 g l^-1 Cu; 200 g l^-1 Zn) and led to a reduction in the accumulation of cadmium and an increase in that of copper and zinc. More specifically, high levels of zinc caused a decrease in cadmium uptake and an increase in copper accumulation. The presence of copper resulted in depressed cadmium accumulation while zinc accumulation increased. Cadmium tended to enhance zinc accumulation, but copper accumulation was only affected to any great extent when zinc was also present.     

Accumulation of cadmium in the mussel Mytilus edulis: kinetics and importance of uptake via food and sea water

In a short-term (162 h) accumulation experiment with mussels Mytilus edulis exposed to 100 ppb Cd and fed algal cells (Phaeodactylum tricornutum) in 1986, it was found that uptake via food played an insignificant role compared to direct uptake from ambient water (19 S). From measurements of the filtration rate and Cd uptake rate, it was estimated that the fraction of Cd taken up per liter of water filtered was about 0.15%. The initial uptake of Cd was linear with time and about three times higher in fed than in starved mussels. From the measured uptake of Cd in starved mussels collected in 1982 and exposed to 10, 100 and 200 ppb in long-term experiments (up to 242 d), it was found that the Cd Accumulation rate was not linear with time, and that the Cd uptake was not directly proportional with the exposure concentration. Values as high as 100 to 1 300 ppm Cd (dry wt of soft parts) were measured. It was found that the Cd elimination rate was not directly proportional with the Cd body burden in long-term exposed mussels, thus indicating that a certain fraction of Cd may have been immobilized to metallothioneins.     

Ultrastructural and cytochemical study on interactions between nutrient storage cells and gametogenesis in the mussel Mytilus edulis

The ultrastructural morphology of storage-cell breakdown and replenishment in Mytilus edulis is described for mussels collected at monthly intervals over a period of one year (September 1981 to October 1982) from a site in Cornwall, England. In addition cytochemical staining reactions at light and electron microscope levels are described for the storage cells and the Sertoli cells. The breakdown of adipogranular (ADG) cell components is believed to be a lysosomally-mediated process of controlled autolysis involving fusion of primary lysosomes with the protein granules. Increasing activity for lysosomal enzymes during ADG cell breakdown is demonstrated using quantitative cytochemistry. The presence of glycogen in vesicular connective tissue (VCT) cells is demonstrated using freeze-dried, formaldehyde-vapour-fixed tissue. The breakdown of VCT cells involves sequestration of glycogen from the central reserve into small cytoplasmic vesicles; subsequent release appears to be mediated by eccrine secretion. The presence of arylsulphatase activity in lysosomes within the peripheral cytoplasm of the VCT cells during the winter months indicates that glycogen breakdown may also be lysosomally mediated. The morphology of the Sertoli cells is described; during the early stages of spermatogenesis they are characterized by the presence of numerous lipid droplets which are depleted as development proceeds. Contact between Sertoli cells and developing spermatogonia is maintained by means of long septate junctions. Following spawning, the well developed lysosomal system is evident in the Sertoli cells and appears to be engaged in intracellular digestion of phagocytosed waste sperm and residual cytoplasm. Resorption of the products of gamete degeneration by the gonoduct epithelial cells is described. During regeneration, VCT cells appear to endocytose material; the ADG cells demonstrate extensive arrays of rough endoplasmic reticulum and are occasionally seen undergoing mitosis. A process involving loss of glycogen vesicles from the mantle epithelial cells by means of apocrine sections is described.     

Growth,condition and specific dynamic action in the mussel Mytilus edulis recovering from starvation

Juvenile Mytilus edulis were grown individually in plastic racks in a tidal salt marsh for 72 d in 1984, starved in the laboratory for 130 d, and then fed the alga Isochrysis galbana daily for 64 d. Oxygen consumption was measured at various times during the course of starvation and recovery. The effects of both size (tissue dry weight) and condition (tissue size relative to shell size) on the rate of oxygen consumption during the course of starvation and recovery were analyzed by multiple regression. Weight-specific preprandial rate was inversely correlated with both size and condition. Weight-specific active rate (measured shortly after feeding) was correlated with size but not condition. Relative Scope for Activity was inversely correlated with size and positively correlated with condition. Relative Specific Dynamic Action (RSDA; the integrated physiological and mechanical response to a meal) was initially correlated negatively with size and subsequently positively with condition. Glycogen content was shown to be positively correlated with condition in mussels before starvation and during recovery. During recovery, experimental mussels returned to 90% of their estimated dry weight prior to starvation, and from 53% after starvation. At weekly intervals during recovery, oxygen consumption was measured following a meal until it returned to the preprandial rate. Both pre- and postprandial volumes (l at STP) of oxygen consumed per hour ( O _2pre and O _2post , respectively) increased significantly during recovery. The postprandial rise in oxygen consumption increased significantly from 15% to 23% of O _2pre . At the end of the recovery period, RSDA [( O _2post – O _2pre )/ O _2pre ] was independent of final dry weight, but was significantly correlated with percent dry weight recovered (r ²=0.44; df=10; P<0.02). The increase in RSDA may reflect increased utilization of food and its conversion to soma during recovery from starvation, as distinct from mechanical energy expenditure (feeding activity) following a meal.     

Contribution to the ecotoxicological study of cadmium,copper and zinc in the mussel Mytilus edulis

A regulation of internal levels of some essential metals has been observed in various animals, whereas the bioaccumulation of several non-essential metals parallels their overloads in water. In the mussel Mytilus edulis L., we have attempted to determine if such a phenomenon exists by comparing the patterns of accumulation of copper and zinc vs cadmium. With this aim, mussels collected in the Bay of Bourgneuf (France) in November 1983 were exposed to these metals for 16 d. At external levels of zinc as high as 100 gl^-1, mussels were able to maintain a normal concentration in all groups of organs for 4 d. The ability of mussels to limit the bioaccumulation of copper and zinc varied from organ to organ, and decreased with higher levels of contamination and longer periods of exposure. In contrast, at the lowest experimental concentration and the lowest period of exposure, a significant increase of cadmium in mussel tissues was generally observed. Even at the highest sub-lethal doses, the levels of copper and zinc in mussel tissues were not much higher than the natural levels (contaminated:background ratios= 2.3 to 6.1), whereas the bioaccumulation of cadmium was less well restricted (contaminated:background ratios=136 to 192). The use of mussels as a bioindicator of pollution seems doubtful for essential metals, particularly as regards short-term pollution, since the levels of these trace elements in the organisms are largely independent of their concentration in the ambient seawater.     

Aspects of nitrogen metabolism of the common mussel Mytilus edulis: Adaptation to abrupt and fluctuating changes in salinity

Mytilus edulis L. were exposed to abrupt (3015 and 1530) and fluctuating (sinusoidal 12 h cycles of 301530) changes in salinity, and the changes in the total osmoconcentration of the haemolymph were recorded. The response of nitrogen metabolism to the altered extracellular osmotic concentrations was investigated in terms of the concentrations of the total NPS (ninhydrin-positive substances) pool and the individual amino acids of the tissues, the concentration of the amino acids of the haemolymph, and the rates of excretion of ammonia and amino acids by whole individuals. The haemolymph became isosmotic with the seawater with abrupt changes in salinity, but with fluctuating salinity was slightly hyperosmotic as the salinity decreased and then slightly hypo-osmotic as the salinity increased. This resulted in a reduction in the extent of the extracellular osmotic change compared to the change in fluctuating salinity to which it was exposed. Total NPS of the tissues decreased with an abrupt decrease in salinity and increased with an abrupt increase in salinity, but a seasonal dependence of the response was indicated. The short-term response of tissue NPS to fluctuating salinity was equivocal, but with long-term exposure the concentration declined. Ammonia and amino acid excretion increased with both an abrupt decrease in salinity and fluctuating salinity and decreased with an abrupt increase in salinity. Haemolymph amino acids increased with an abrupt decrease in salinity. The increased rates of nitrogen excretion accounted for the reductions in the NPS concentrations of the tissues except in the early stages of fluctuating salinity. Taurine, aspartate, threonine, serine, glycine and arginine declined with an abrupt decrease in salinity while alanine and glutamate increased slightly. With an abrupt increase in salinity, alanine and ammonia accumulated in the tissues and then declined while the other amino acids increased slowly over a longer time-course. Similar individual amino acid responses were seen with long-term exposure to fluctuating salinity, except for taurine which did not decrease in concentration. On the basis of the changes in tissue amino acids and ammonia, it is suggested that the alanine dehydrogenase reaction is the primary nitrogen-fixing reaction in marine bivalves such as M. edulis.     

Some aspects of nitrogen metabolism in Mytilus edulis: effects of aerial exposure

The effects of starvation and emersion on the ammonia effluxes of Mytilus edulis L. were studied. Both fed and starved groups showed similar patterns of efflux during re-immersion, indicating no compensation for starvation during anaerobic catabolism; this is taken as evidence that carbohydrate or lipid is the predominant energy source at the outset of anaerobiosis. Instead of an overshoot of ammonia excreted during the 4 h following re-immersion, all groups showed significantly reduced efflux rates independent of emersion duration. Such results suggest some conversion of ammonia during the recovery period, as no significant drop in haemolymph ammonia occurred at this time. The level of ammonia accumulation in haemolymph and mantle cavity fluid decreased exponentially with emersion duration, implying the stabilisation, at a low rate, of ammonia production after a relatively long period of emersion (16 to 24 h). Oxygen levels of the mantle cavity fluid dropped rapidly during emersion but never attained anoxia over 24 h of emersion, which may indicate some oxygen uptake by emersed mussels. It has been concluded that the behaviour and physiological responses shown by M. edulis to emersion appear to reflect a need for respiratory gas exchange more than a response to desiccation – presumably in the interest of energy-conservation. The reductions in ammonia efflux rate during emersion and, to a certain extent, immediately after re-immersion have also been considered here to be energy-saving strategies that illustrate the importance of ammonia in restoring normal levels of some amino acids following re-immersion. Received: 26 August 1997 / Accepted: 27 May 1999