The filtration rate of Mytilus edulis and its dependence on algal concentration,measured by a continuous automatic recording apparatus

A new apparatus for long-term, continuous automatic measurements of filtration rates in suspension-feeding organisms is described. As the concentration of algae in the experimental medium is diminished by the filter-feeding activity of the experimental animals, algal suspension is automatically added, thus keeping the algal concentration constant. In this way, accurate determinations of filtration rates in relation to particle concentration are made possible. For determination of filtration rates in the common mussel Mytilus edulis L., individuals of different body size (shell length 8.5 to 56.5 mm) were used. Within the range of 10x10⁶ to 40x10⁶ cells of Dunaliella marina/l, mussels of the same body size filter-out approximately the same amount of algae at high or low concentrations. A low algal concentration is counterbalanced by a corresponding higher filtration rate. Within the range of body size (W=dry weight of tissues) and algal concentrations used, the filtration rate (F) follows the general allometric equation F=a·W ^b, where a and b are constants at specific experimental conditions. At a temperature of 12 °C, the values obtained for a are 2410 at a concentration of 20x10⁶, and 1313 at a concentration of 40x10⁶ Dunaliella cells/l; correspondingly, the filtration rates of a mussel of 1 g dry-tissue weight are 2410 ml/h and 1313 ml/h. b, the slope of the regression line (0.73 to 0.74), is independent of algal concentration. However, examination of all known measurements reveals that, most probably, the general allometric equation is an oversimplification; in large individuals there is a more pronounced decrease in filtration rate. The relationship between filtration rate, body size of mussels, and algal concentrations used is discussed.This work was made possible through a research grant from the Deutsche Forschungsgemeinschaft in connection with the program Litoralforschung — Abwässer in Küstennähe.     

An improved automatic recording apparatus for determining the filtration rate of Mytilus edulis as a function of size and algal concentration

An automatic recording apparatus for measuring the filtration rate in suspension-feeding bivalves is described. The concentration of algae in the experimental medium is kept constant throughout each experiment by addition of Phaeodactylum tricornutum from a chemostat. Within the range of body size 5.7 to 283 mg (W=dry weight of tissues), the filtration rate (F=ml min^-1) at 15°C in Mytilus edulis L. follows the allometric equation F=0.85 W ^0.72. Within the concentrations 0.18 to 0.70 mg algal dry weight l^-1, the filtration rate in mussels of 132 mg dry flesh weight ranges from 33.1 to 41.0 ml min^-1. At 0.18 mg algal dry weight l^-1 the mussels filter continuously for 20 h, with a high constant rate that presumably represents the water transport capacity under optimal laboratory conditions.     

Energy budgest,growth and filtration rates in Mytilus edulis at different algal concentrations

Growth of Mytilus edulis L. was measured in aquaria with through-flowing sea water at different levels of constant algal concentrations. The amount of food and oxygen consumed by the mussels were measured over given periods as well as the changes in dry organic weight during the same periods. From these parameters it was possible to make simple energy budgets and to compare the estimated growth with actual growth, and, further, to determine growth efficiences at different food levels. Energy budgets were made for mussels grown at algal concentrations of 0, 1.6×10³, 3.0×10³ and 26.0×10³ Phaeodactylum tricornutum cells x ml^-1. The estimated growth was found to be close to actual growth at algal concentrations above maintenance level and the net growth efficiency was found to be between 18% (3.0×10³ cells x ml^-1) and 61% (26×10³ cells x ml^-1). It has been shown that the filtration rate is independent of algal concentrations between about 1.5×10³ to 30×10³ P. tricornutum cells x ml^-1. Outside this range a decrease in filtration rate was noticed.     

Effect of temperature and food on the heart beat,ventilation rate and oxygen uptake of Mytilus edulis

The heart beat, ventilation rate and oxygen uptake of Mytilus edulis L. were measured simultaneously, in response to changes in temperature and food level. There was no thermal acclimation of heart-beat frequency or amplitude to temperatures from 5° to 25°C. Oxygen consumption and ventilation rate acclimated to 10°, 15° and 20°C, but not to 25°C. Starvation reduced the rate of oxygen uptake and heart-beat frequency to a standard level and, in response to food, the ventilation rate and oxygen consumption immediately increased to an active level. Feeding was maintained after the initiation of active metabolism, and during the following 10 days the heart-beat frequency gradually increased to the level characteristic of fed individuals. There was no direct correlation between the rate of oxygen consumption and heart rate, and an apparent absence of a close nervous coupling between ventilation rate and heart rate in M. edulis.     

The combined effects of salinity and temperature on larvae of the mussel Mytilus edulis

The combined effects of salinity and temperature on survival and growth of larvae of the mussel Mytilus edulis (L.) were studied. The effects of salinity and temperature are significantly related only as the limits of tolerance of either factor are approached. Survival of larvae at salinities from 15 to 40 is uniformly good (70% or better) at temperatures from 5° to 20°C, but is reduced drastically at 25 °C, particularly at high (40) and low (20) salinities. Larval growth is rapid at a temperature of 15 °C in salinities from 25 to 35, at 20 °C in salinities from 20 to 35. Optimum growth occurs at 20 °C in salinities from 25 to 30. Growth decreases both at 25° and 10 °C; the decline is most drastic at high (40) and low (20) salinities.Part of a study completed at the Bureau of Commercial Fisheries, Biological Laboratory, Milford, Connecticut, USA, while on a UNESCO Fellowship.     

Effects of temperature,nutritive and metal stressors on the reproductive biology of Mytilus edulis

Experimental observations suggest that the gametogenic cycle of Mytilus edulis from Swansea Bay, South Wales (U.K.) is adversely modified by the combined effects of temperature, nutritive and sublethal metal-induced stress. During low temperature stress, oogenesis remained in the vegetative stage and vitellogenesis only commenced when temperatures were raised to ambient levels. Copper was the most toxic metal, although its uptake was slowest. A concentration of 0.05 mg l^-1 of this metal proved lethal at ambient temperatures, although at low temperatures there was no mortality. Copper suppressed both the growth of young oocytes and vitellogenesis in larger oocytes. Zinc was less toxic than copper, although this also inhibited oocyte development and resulted in severe lysis of gametes. Cadmium was the least toxic of the three metals studied and suppressed gametogenesis only in the initial stages of gonad development. The low toxicity of cadmium to gametogenesis may be a result of its high solubility and its possible storage in the digestive gland.     

Effects of oil pollution on the freezing tolerance and solute concentration of the blue mussel Mytilus edulis

Exposure of blue mussels (Mytilus edulis L.) to oil-polluted sea water for 7 d in the laboratory did not affect their body fluid concentrations of inorganic ions and free amino acids. Mussels exposed to-4°C for 12 h did not freeze, whereas freezing occurred invariably in mussels exposed to-7°C or lower temperatures for the same period. Following freezing at-10°C, oil polluted mussels resumed normal activity considerably more slowly than unpolluted ones, but oil-polluted as well as unpolluted mussels showed normal activity 3 h after thawing. Freezing at-15°C was invariably lethal to individuals of both groups. One possible explanation of the delayed recovery of oil-polluted mussels frozen at-10°C may be that oil components had become concentrated to toxic levels as the amount of solvent water diminished during freezing.     

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     

Influence of marine microalgal metabolites on the feeding behavior of the blue mussel Mytilus edulis

Effects of microalgal ectocrines on the feeding behavior of the blue mussel Mytilus edulis L., collected from Delaware Bay, Lewes, USA, were studied. Unialgal cultures of six species of marine microalgae were grown to the end of their exponential growth phase. Cells were removed by gentle filtration, leaving dissolved microalgal ectocrines in culture filtrates. Filtrates were then tested in filtration rate and particle selection bioassays. In filtration rate bioassays, dissolved ectocrines were delivered to mussels in a flow-through apparatus and removal of beads (6 m) used to determine rates. Filtration rates were significantly reduced by dissolved ectocrines from two of the microalgal species tested (Olisthodiscus luteus Carter and Dunaliella tertiolecta Butcher). None of the microalgal filtrates stimulated filtration rates of mussels. In particle selection bioassays, ectocrines were adsorbed onto either reverse phase (lipophilic) or normal phase (hydrophilic) microparticles (10 m) and delivered to mussels with an equal concentration of control treated particles. Mussels showed significant selection, either rejection or preferential ingestion, for ectocrine treated spheres depending upon microalgal species and sphere type used. This research shows that pre-ingestive chemical cues from microalgae can influence mussel feeding behavior. We suggest that epicellular ectocrines are more important in mediating this behavior than dissolved ectocrines.     

Morphological variation of Mytilus edulis and Mytilus trossulus in eastern Newfoundland

Allopatric populations of Mytilus species show distinct shell morphology which may be due to genetic and/or environmental effects. Sympatric populations of Mytilus species show similar shell morphology which may be due to hybridization eroding morphological differences and/or the influence of common environmental conditions. The present study examined shell morphology and shell shape from 16 sites in eastern Newfoundland where M. edulis L. and M. trossulus Gould coexist in common environments with limited hybridization. Shell morphology was based on measurements of eight characters, and shell shape was quantified by elliptic Fourier analysis of shell outlines. Significant differences were observed between species for both shell morphology and shell shape across 16 sites sampled. The relatively small differences in morphology and shape between the species were probably due to exposure to common environments rather than hybridization. Shell shape for M. edulis was more eccentric compared to M. trossulus which was more elongated. Shell shape analysis of a range of size classes at one site showed a change from an eccentric to an elongated shape going from the smaller to the larger size classes. Both species showed a similar trend, with the larger M. edulis more eccentric and the larger M. trossulus more elongated. Received: 17 July 1998 / Accepted: 6 January 1999     

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.     

Degradation of bacteria by Mytilus edulis

The uptake of several species of bacteria by the common mussel Mytilus edulis (L.) and the subsequent fate of some polymers of the bacteria have been investigated in a study carried out during 1981. Bacteria (Escherichia coli, Micrococcus luteus, M. roseus, Bacillus cereus, Staphylococcus aureus and a marine pseudomonad, 1-1-1) were radiolabelled by growth in medium containing ³H-thymidine and the uptake of bacteria by Mytilus edulis was monitored. Labelled and unlabelled bacteria, at initial concentrations of 0.5 to 1x10⁷ bacteria ml^-1, were cleared at similar, exponential rates with no significant difference in the rates for different bacteria: 90% of bacteria were cleared in a mean time of 1.93±0.12 h (SEM, n=63). Those bacteria with cell walls which were sensitive to M. edulis lysozyme were rapidly degraded by the mussel and ³H-labelled DNA was released in a form not precipitable by 10% trichloroacetic acid. Lysozyme-resistant bacteria (Micrococcus roseus and S. aureus) were cleared from suspension by Mytilus edulis but most were rejected intact. By measuring the rate of release of ³H-thymidine-labelled material from the mussel the rate of degradation of lysozyme-sensitive bacteria by M. edulis was found. For different bacteria the degradation rate varied from approx 2x10⁸ to 27x10⁸ bacteria h^-1 with an overall mean of 10x10⁸ bacteria h^-1. A thymidine- and diaminopimelicacid-requiring auxotroph of E. coli was radiolabelled with ³H-thymidine, ³H-diaminopimelic acid or ¹⁴C-glucose and fed to M. edulis. Bacteria were cleared and degraded by the mussel; ³H-diaminopimelic acid-labelled or ¹⁴C-glucose-labelled polymers were retained, whereas ³H-thymidine-labelled polymers were released into the surrounding water. Extracts of the digestive gland of M. edulis degraded lysozyme-sensitive bacteria to release ³H-thymidine-labelled material, but did not release ³H-thymidine-labelled material from lysozyme-resistant bacteria. It is concluded that M. edulis can select lysozymesensitive bacteria for subsequent processing and discriminate between bacterial polymers to reject DNA. Also, bacteria could provide a substantial fraction of the carbon requirement of the mussel.     

Influence of food concentration on the physiological energetics and growth ofOstrea edulis larvae

Feeding, respiration and growth rates of oyster (Ostrea edulis L.) larvae reared at five food levels were measured throughout the entire larval period. Energy budgets were derived as a function of alga (Isochrysis galbana Parke) food concentration. Ingestion rate (IR, cells h^-1) and oxygen consumption rate ( , nl h^-1) were almost isometric functions of larval size [ash-free dry weight, (AFDW, g)], characterized by the equations: IR=803.9 AFDW^1.13 and =4.85 AFDW^1.09. Ingested ration was directly correlated to cell concentration up to a maximum at 200 cells l^-1, with further increases failing to support higher ingestion rates. Likewise, growth rate linearly increased with food ration up to 100 cells l^-1 (max. growth efficiency,K ₁=25%) and reached a maximum at 200 cells l^-1 (growth rate=5.6 m d^-1), with further increases in food not supporting significantly faster growth. Maintenance ration was 2 to 3% daily dry weight (DW); optimum ration increased during larval development from 5 to 20% DW; maximum ration was 20% DW. During larval rearing, an increasing feeding schedule of 50, 100 and 200 cells l^-1 from Days 0, 5 and 10, respectively, is recommended.     

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.     

Influence of food quantity on the kinetics of cadmium uptake and loss via food and seawater in Mytilus edulis

The difference between the cadmium uptake via food and seawater in Mytilus edulis has been studied. This was done by labelling algae with Cd-109 and seawater with Cd-115m. Mussels were fed on six different quantities of Isochrysis galbana. Cadmium uptake via algae was more efficient at low food levels, while accumulation from seawater was linearly correlated with food quantities. Cadmium from food contributed only little to the body burden (0.2–0.5%). Half-lives for the elimination of cadmium ranged from 96–190 d and increased with decreasing availability of algae, presumably due to slowed down metabolism. Differences in elimination patterns suggest a release of both isotopes from different storage depots. A computer model shows that the food pathway can only play a significant role if algae are highly contaminated. It also demonstrates the paradox that in long-term studies the highest contribution of food-derived cadmium to the body burden must be expected near maintenance food concentrations.     

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.     

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.     

Temporal patterns of shell-gape in Mytilus edulis

Hitherto published evidence of the presence or absence of endogenous activity rhythms in bivalve molluscs is equivocal. Mytilus edulis L. were collected from a North Wales (UK) estuary in 1985, and shell-gaping was investigated in individual mussels under constant conditions in the laboratory. Results suggest that there is no endogenous circatidal rhythm of shell-gaping in this species, This is consistent with the view that, unlike mobile species, sessile intertidal species are much more likely to exhibit exogenous rather than endogenous responses to tidal fluctuations. There is some evidence of weak circadian rhythmicity of shell-gaping in M. edulis, with greater duration of shell-closure during hours of expected daylight. Such behaviour could represent an adaptational defence against visually-feeding predators.     

Total body zinc concentration and allometric growth ratios in Mytilus edulis collected from different shore levels

Body zinc concentrations (on a soft parts dry weight basis) in 18 composite collections of mussels (Mytilus edulis) taken from different shore levels within a single mussel bed on the north shore of the Tyne estuary at Tynemouth, on 25 October, 1980, showed highly significant correlations with 4 allometric, size-related factors. A negative correlation was found between body zinc concentration and a flesh condition (FC) index defined as mg dry flesh wt per g dry shell wt. Positive correlations were found between body zinc concentration and width: height ratio, width: length ratio and shell valve inequality index (SVI) defined as the weight of heavier shell valve/weight of lighter shell valve. Unlike earlier observations on a single population taken from a single habitat, this heterogeneous collection showed no significant relationship between body zinc concentration and body size as determined by soft-parts dry weight. Body zinc concentration and allometric ratios were, however, related to position on the shoreline. It is suggested that, where mussels are collected from different habitats, environmental factors probably have a greater influence on body size than age. Therefore, normalization of trace metal concentrations to a standard body weight may not be appropriate where specimens are collected from a variety of habitats. Allometric ratios are related to age, but are independent of absolute size, and may therefore provide a more suitable basis for normalization of trace metal concentration within a monitoring programme.Contribution No. 1222 of the Center for Estuarine and Environmental Studies of the University of Maryland     

Uptake and cellular distribution of cadmium in Mytilus edulis

Cadmium uptake has been studied in starved and fed Mytilus edulis L. It is suggested that fairly elevated cadmium contents in fed mussels are not due to contaminated food, but to increased pumping rate when food is available. Highest concentration and main body burden are found in the mid-gut gland. Transport via haemolymph, and selective discrimination at the basement lamina of the mid-gut gland tubuli are regarded as mainly responsible for accumulation. Mercury seems to be processed in a similar way as cadmium. In the tubuli, both metals are immobilized in membrane-bound vesicles, which are finally defaecated.This work was financially supported by the German Research Council (DFG) (Sto 75/4 and Th 158/13).