Assimilation of trace elements by the mussel Mytilus edulis: effects of diatom chemical composition

Mussels have been widely used as bioindicators of coastal contamination, and recent reports have demonstrated that metals are accumulated from both the dissolved phase and from ingested food. In the winter and spring of 1995, we examined the influence of the chemical composition of food (protein content, trace element concentrations and ratios in the diatom Thalassiosira pseudomana) on the assimilation of six trace elements (Ag, Am, Cd, Co, Se and Zn) in the mussel Mytilus edulis (L.). Differences of up to 38% in diatom protein content had no major influence on the assimilation of any trace element or carbon. Protein assimilation in M. edulis examined with a ³⁵S radiotracer was also independent of protein content in the diatoms. Similarly, Se assimilation in mussels was not affected by the different Se concentrations in the diatoms. Cd assimilation increased with increasing Cd concentration, presumably due to higher desorption of Cd under acidic conditions typical of the mussel gut. Zn assimilation was inversely related to Zn concentration in the food particles, implying a partial regulation of this metal in the mussels. There was no evidence of any interaction of Cd and Zn in their assimilation by the mussels. These results suggest that mussels are highly responsive, in an element-specific way, to some components of ingested food (e.g., metal concentration), but other food components (such as the biochemical composition of the algae) have little effect on assimilation.     

Reproductive isolation and reproductive output in two sympatric mussel species (Mytilus edulis, M. trossulus) and their hybrids from Newfoundland

The mussels Mytilus edulis L. and M. trossulus Gould are found sympatrically in most areas of Newfoundland, with a low frequency of hybrids. To assess the potential for reproductive isolation, we sampled mussels from three sites in an eastern Newfoundland Bay from May–October 1996 to determine if there were differences in the reproductive cycles of the two species and their natural hybrids. In mussels with sheil lengths of 38–42 mm, males and females with mature gametes were dominant in June for M. edulis and hybrids, while M. trossulus showed a lower frequency of individuals with mature gametes. M. trossulus and hybrids spawned over a prolonged period (from late spring to early autumn) compared with most M. edulis individuals that spawned over a period of 2–3 weeks in July. This asynchrony in spawning activity between the two species may partially explain the low frequency of hybrids found in previous studies of these mussel populations. Female and male hybrids between M. edulis and M. trossulus showed normal gonad development, ripening and spawning, providing an opportunity for the introgression of genes between the two species. M. trossulus had a higher reproductive output than M. edulis of similar shell length, while hybrids showed intermediate values of reproductive output. M. trossulus females produced smaller eggs than either M. edulis or hybrids. Differences in reproductive traits may partially explain the maintenance of the mussel hybrid zone in Newfoundland. Published online: 13 August 2002     

Genotype-dependent fecundity and temporal variation of spawning in hybrid mussel (Mytilus) populations

Mussel samples were collected at 4 to 6 wk intervals throughout 1987 from two hybridMytilus edulis/M. galloprovincialis populations, at Croyde Bay and Whitsand Bay, in southwest England. These were analyzed at two polymorphic loci which are diagnostic for allozyme differences which typifyM. edulis andM. galloprovincialis. Dry mantle weight as a function of shell length was determined for all individuals of each sample. Size-frequency data for the two populations was obtained in September 1987 and March 1988. For all genotypes at both sites, fecundity was a function of shell length, and in both populations the frequency ofM. galloprovincialis alleles was positively correlated with shell length. At both sites, allozyme genotype explained a significant amount of variation in mantle weight either when assessed as a main effect or when assessed as an interaction with shell length or time of collection. At Croyde,M. galloprovincialis mussels had greater estimated fecundity per unit length than theM. edulis mussels. Differences in the timing of spawning activity between theM. edulis and theM. galloprovincialis mussels were inferred, and these differences might act to reduce the amount of interbreeding at Croyde. At Whitsand, a reduced level of variability in the timing of spawning activity and fecundity between the genotypes was observed and explained by a higher degree of genetic mixing. Because theM. galloprovincialis mussels had (1) a greater estimated fecundity at any length, and (2) a greater mean length than theM. edulis mussels, the mean genotypic annual fecundity perM. galloprovincialis mussel was 2.8 times greater than an individualM. edulis mussel at Croyde, and 2.2 times greater than an individualM. edulis mussel at Whitsand. This evidence thatM. galloprovincialis mussels have an advantage in fecundity, and thus perhaps in fertility, taken together with the evidence thatM. galloprovincialis also has a higher viability, indicates directional selection in favour of theM. galloprovincialis phenotype. Because of the observed temporal stability of the population it seems likely that this selection is counterbalanced by a massive imigration ofM. edulis spat from neighbouring populations.     

Genetic variation of mitochondrial DNA in mussel (Mytilus edulis and M. galloprovincialis) populations from South West England and South Wales

Mitochondrial DNA (mtDNA) and allozyme variation were analysed in samples of mussels collected in 1984 and 1985 from four localities in South West England and one locality in South Wales, a region of Britain where the common mussel (Mytilus edulis) occurs sympatrically and hybridises with the Mediterranean mussel (M. galloprovincialis). Significant differences in mtDNA genotype frequencies for three restriction enzymes (BstEII, XbaI, and EcoRI) were observed between mussels from M. galloprovincialis populations (Padstow and Bude) and those from an M. edulis population (Swansea). Some mtDNA genotypes at high-frequency in M. galloprovincialis were not observed in M. edulis, although there was no indication that mtDNA variation provides greater overall diagnostic power than allozyme variation in distinguishing between the two forms of mussel. Construction of a phylogenetic tree of multiple mtDNA genotypes revealed small mutational distances between the genotypes characterising M. edulis and M. galloprovincialis. The results were consistent with predominant mtDNA flow from M. edulis to M. galloprovincialis. This can be explained by the dispersal of larvae to South West England from M. edulis regions to the north and east, but little dispersal in the opposite directions. Samples from two hybrid populations (Whitsand and Croyde) were analysed. mtDNA genotype frequencies at Croyde were in line with predictions made on the basis of two partially diagnostic allozyme loci (Est-D and Odh), mtDNA frequencies at Whitsand were not. Frequencies of some mtDNA genotypes at Whitsand were characteristic of M. edulis, others of M. galloprovincialis. Differential selective mortality or flow of different mtDNA genotypes and allozyme variation are proposed as possible causes of these results.     

Variation in strength of attachment to the substrate explains differential mortality in hybrid mussel (Mytilus galloprovincialis and M. edulis) populations

Mussel samples were collected from a hybrid mussel (Mytilus galloprovincialis and M. edulis) Population at Croyde, southwest England, in January, March and May 1990. The strength of attachment of each mussel to the substrate was measured with a spring balance. A number of diagnostic characters were also recorded. These are shell lengh, width and height, mantle colour and genotype at two allozyme loci, esterase-D and octopine dehydrogenase. Multiple-regression analysis was used to assess the effect of the diagnostic characters on strength of attachment as dependent variable. Mussels possessing the relatively high shells and darker mantle colouration characteristic of M. galloprovincialis had higher values, on average, for strength of attachment than mussels resembling M. edulis. Phenotypically intermediate mussels had intermediate values for strength of attachment. The results suggest an adaptive difference which can account for reports of differential mortality acting in favour of M. galloprovincialiis.     

Quantitative studies on gonad cycle,fecundity, reproductive output and recruitment in a baltic Mytilus edulis population

Since Mytilus edulis L. is a biomass dominant in the Baltic much interest is focused on the ecology of the species. In this paper an attempt is made to quantitatively cover the reproductive cycle of a Baltic M. edulis population in order to provide data for energy flow models and to discuss aspects of recruitment in this species. Histological preparations of gonads showed that gametogenesis started with declining temperatures in autumn and proceeded very slowly through winter. At the beginning of March when food was supplied during the spring phytoplankton bloom, rapid maturation took place. This was also revealed by an increase in meat weight of the mussels. Only one spawning period was recorded, from the middle of May until the beginning of June, due to food being strongly limited to the population during the rest of the year. The length of the larval period was estimated as being 5 to 6 wk and settlement was registered from the end of June through July. In general the large annual variations found in the Baltic with regard to temperature and food abundance give rise to a more marked annual pattern in the reproductive cycle than is encountered in other seas. Fecundity was assessed for two populations from 4-and 15-m depths from studies covering two annual cycles of the changes in the relation of shell length — meat weight. The size-related fecundity was found to be equal in both populations and related to food abundance and not to growth or age. Fecundity, expressed as weight loss at spawning, ranged from 0% in 2-mm mussels and increased from 38 to 52% in 10-to 30-mm mussels. The fecundity as percentage of biomass in full-grown Baltic M. edulis is of similar magnitude as in full-grown mussels from other areas despite the smaller size of Baltic mussels. The reproductive output for the total 160 km² research area was calculated as being 1 200 tons dry weight or 80% of the standing stock, which, due to the particular features of the Baltic M. edulis population probably represents the larger part of the total mussel production. This reproductive output, calculated as 8·10⁷ eggs·m^-2 and corresponding to 50% of the total annual zooplankton production, may thus consitute an important food source for herring larvae and carnivorous zooplankton. Recruitment was divided into two phases: (1) Recruitment of juveniles (=settlement of larvae), and (2) recruitment to breeding stock. Monitoring studies of settlement on ropes and the year round presence of high abundances of mussels <2 mm indicate that settlement is in excess of the demands for maintaining population size and that most settled mussels form a pool of competitively suppressed non-growing individuals. Not until death of an already established mussel will these become recruited to the breeding population. Thus recruitment is possible throughout the year which stabilizes the population and maintains it near the carrying capacity of the area with regard to food and space availability.     

Geographic variation,niche breadth and genetic differentiation at different geographic scales in the mussels Mytilus californianus and M. edulis

Genetic differentiation was investigated in the marine mussels Mytilus californianus Conrad and M. edulis Linn. from the west coast of North America. In allopatry with M. californianus, M. edulis occurs throughout the intertidal zone; however, in microgeographic sympatry its ecological range is restricted to above the M. californianus mussel bed and to patches of substratum opened by natural disturbances within the bed. Over the same geographic scale, the broader-niched M. edulis shows greater among-locality genetic difference and greater levels of polymorphism than M. californianus at two enzyme loci. Genetic differentiation on a geographic basis was investigated in M. californianus at a single rock (on a scale of meters), on an island (on a scale of kilometers), throughout a strait (on a scale of 10² kilometers), and along the west coast of North America (on a scale of 10³ km). Differentiation was minimal over the west coast, and could be explained by microhabitat differences in a local area. The minimal differentiation in west coast mussels relative to strong geographic differentiation of M. edulis on the east coast of North America may be related to the steeper latitudinal thermal gradient on the east coast. Local ecologically related microgeographic variation can result in biased and misleading estimates of genetic heterogeneity but microgeographic variation at enzyme loci may not be due to selection at the loci investigated or may even be due to the breeding structure of the mussel populations, as suggested by Tracey et al. (1975).     

Effects of substrate type on growth and mortality of blue mussels (<Emphasis Type="Italic">Mytilus</Emphasis><Emphasis Type="Italic">edulis</Emphasis>) exposed to the predator <Emphasis Type="Italic">Carcinus</Emphasis><Emphasis Type="Italic">maenas</Emphasis>

Structure and complexity of the substrate are important habitat characteristics for benthic epifauna. The specific growth and mortality rates and inducible defence characters on medium-sized blue mussels (Mytilus edulis L.) exposed to shore crabs (Carcinus maenas L.) were examined on three different substrate types in combined field and laboratory experiments. The experiments showed that complexity of the substrate increased blue mussel survival significantly, through a decrease in predation pressure. However, increased intraspecific competition for food on the complex substrate resulted in significantly lower growth rates of the mussels. Inducible defence characters were also influenced by substrate type. Blue mussels were more affected by predators on the structurally simple substrate, where they developed thicker shells and a larger posterior adductor muscle.     

Roach (<Emphasis Type="Italic">Rutilus rutilus</Emphasis>) as an important predator on blue mussel (<Emphasis Type="Italic">Mytilus edulis</Emphasis>) populations in a brackish water environment,the northern Baltic Sea

Suspension-feeding bivalves are organisms of major functional importance in several aquatic environments around the world. They are also important food items for many fish and benthivorous seabirds. It has commonly been thought that predation pressure on blue mussel (Mytilus edulis) populations is negligible in the Baltic Sea, owing to the scarcity of major invertebrate predators such as starfish and crabs. It has recently been shown, however, that the blue mussel is the main food item for roach (Rutilus rutilus) in the archipelago areas of the western Gulf of Finland, where this freshwater fish species has become increasingly abundant, mainly due to increased eutrophication. To quantify the influence of roach predation on blue mussel populations we measured the standing biomass and size structure of the local blue mussel population and used a bioenergetic model to estimate mussel consumption by individual roach during two consecutive summers, 1997 and 1998. The results of the model were combined with existing data on roach abundance, giving annual consumption estimates of 75–105 kg blue mussel dry weight ha^–1 in the study area, approximately two-thirds of these consumed mussels being >10 mm. This corresponds to approximately one-third of the standing population of mussels >10 mm in the area. Our results suggest that the predation effects of vertebrates on Baltic blue mussel populations are not insignificant, as previously believed. Predation by roach and other predators may have an important structuring effect on unstable blue mussel communities within the Gulf of Finland, where the species lives at the edge of its range.Communicated by M. Kühl, Helsingør     

Effect of a tripeptide on the aggregational behaviour of the blue mussel<Emphasis Type="Italic"> Mytilus edulis</Emphasis>

Aggregation of blue mussels (Mytilus edulis) is stimulated by environmental chemical stimuli. Experiments carried out in a basin with a one-way current showed that individual mussels were attracted to upstream mussel concentrations and moved actively in their direction. The involvement of a tripeptide in this migration was implicated by experiments demonstrating that individual mussels were effectively attracted and moved actively towards a source of glycine–glycine–arginine at concentrations of 0.56–3.78×10^–10 M. A distinct seasonal difference in the extent of movement towards mussel concentrations was found. From the beginning of autumn, movement decreases linearly towards zero movement in winter.     

The relationship between allele frequency and tidal height in a mussel hybrid zone: a test of the differential settlement hypothesis

The blue mussels Mytilus edulis L. and M. galloprovincialis Lmk. hybridize in western Europe. Within hybrid populations nuclear alleles specific to M. galloprovincialis increase in frequency with age and size. This relationship changes with tidal height; alleles from M. galloprovincialis occur more frequently high in the intertidal zone, while M. edulis alleles predominate in the low intertidal zone. We tested the hypotheses that larvae with M. galloprovincialis alleles tend to settle higher in the intertidal zone, or that mussels redistribute themselves with respect to tidal height after initial larval settlement. We sampled recently metamorphosed mussels every 2 weeks in a hybrid mussel population at Whitsand Bay in southwest England throughout the summer of 1996. We observed four cohorts of newly settled mussels. There was no evidence of differential settlement of mussels with different genotypes in connection with tidal height, or into shaded versus unshaded microsites. Therefore, we rejected the preferential settlement hypothesis. There was substantial movement of juvenile mussels in the first 4 weeks following initial settlement, but this “secondary settlement” did not result in genetic differentiation with respect to tidal height. Further, significant differences in allele frequencies were found between primary and secondary spat. This allele frequency change was in the opposite direction of that seen in the adult population, suggesting newly settled larvae may be experiencing different selective pressures than adults. We propose that the genetic structure of hybrid mussel populations with respect to tidal height is the consequence of differences in selection intensity. Received: 30 April 1999 / Accepted: 5 May 2000     

Effect of environmental factors on byssal thread formation

The effects of various factors on byssal thread formation have been examined in the laboratory using Modiolus demissus and Mytilus edulis. With M. demissus thread formation (threads/mussel/h0 and proportion of mussels forming threads decreased with increasing size; prior exposure to air enhanced subsequent thread formation; mechanical agitation reduced thread formation. In addition, low salinity acclimated mussels adapted more rapidly to 32 ppt than high salinity acclimated mussels did to 16 ppt; threads were not formed in the absence of calcium and/or magnesium; and there was no reduction in thread formation at temperatures as high as 27° to 28°C. M. edulis died and thread formation approached zero at temperatures exceeding 26°C.     

Genetic structure of hybrid mussel populations in the west of Ireland: two hypotheses revisited

In Ireland, mussels on exposed rocky shores constitute an interbreeding mixture of two forms of mussels, the blue mussel, Mytilus edulis, and the Mediterranean mussel, M. galloprovincialis. Results from an Irish study in the 1980s, using partially diagnostic allozyme markers, indicated that mussels higher up the shore were more galloprovincialis-like than those lower down. In this study we set out to test two hypotheses: (a) recruits arriving on the shore are composed of genetically distinct cohorts that settle preferentially at different levels on the shore, and maintain genetic distinctiveness into adulthood; (b) recruits are genetically homogeneous, but once settled they diverge genetically over time, due to within-habitat site specific-selection. The diagnostic Me 15/16 DNA marker was used to analyse the genetic composition of newly-settled spat recruiting to artificial substrates, which were placed at two-week intervals from May–October 2002, on the mid- and low shore areas of two exposed sites in Galway Bay. Adult mussels were also collected on each sampling date. Results did not support the preferential settlement hypothesis, i.e., the genetic composition of primary settlers (≤ 500 μm) was similar between tidal heights and shores. Neither was there evidence of post settlement selective mortality, as adults were genetically similar to settling spat. In spat and adults the frequency of the M. galloprovincialis allele was high (0.56–0.80), due to high frequencies of M. galloprovincialis (> 37%) and hybrid (> 33%) genotypes, and correspondingly low frequencies of the M. edulis genotype (< 11%). Adult mussels from a nearby sheltered estuarine site, while significantly different to exposed shore mussels, still had low frequencies of the M. edulis genotype (< 17%), indicating no apparent advantage for the genotype in this environment. There are indications that the genetic composition of mussels may be changing on the Atlantic coasts of Ireland.     

Mussels flexing their muscles: a new method for quantifying bivalve behaviour

We employed a novel technique to quantify how blue mussels Mytilus edulis react to predation risk in their environment by quantifying mussel gape using a Hall sensor attached to one shell valve reacting to a magnet attached to the other. Change in gape angle per second (CHIGA) versus gape angle plots resulted in a distribution with a boundary, which defined the maximum CHIGA of a mussel at all gape angles. CHIGA boundary plots for all individual mussels were similar in form. However, the CHIGA boundary increased in extent with mussel length (maximum CHIGA for mussel valve closures for mussels 2.98 and 79.6 mm long were −1.5 and −11°s⁻¹, respectively), showing that larger mussels opened and closed most rapidly. Mussel extract added to the seawater, a factor believed to signal predation, caused mussels to close significantly faster than otherwise (P < 0.001). This approach for assessing how mussels react to their environment indicates that mussel response to predation is graded and complex and may well indicate animal-based assessments of the trade-off between effective feeding and the likelihood of predation.     

Influence of algal concentration and temperature on the filtration rate of Mytilus edulis

The filtration rates of Mytilus edilis (=galloprovincialis; 40 mm) were determined in relation to food concentration and temperature, using pure suspensions of the unicellular alga Platymonas suecica in concentrations ranging from 3x10⁵ cells/l to 1.5x10⁸ cells/l. The rate of filtration (ml/h/mussel) generally decreased as cell concentrations increased, and dropped to low values when concentrations above 5x10⁷ cells/l were supplied. The amount of water swept clear varied continuously, and noticeable differences in the filtration activity of M. edulis were observed over short time intervals (5 min). Fluctuations of filtered volumes per unit time were greater with lower than with higher concentrations of algae. The influence of temperature on filtration activity was highest between 5°–15°C and 25°–30°C. A temperature increase from 15° to 25°C resulted in only a slight increase in filtration rate. At 5° and 30°C, filtration dropped to very low values, namely 350 and 100 ml/h, respectively. The temperature coefficients for the filtration rates of M. edulis were determined as: Q₁₀ (5° to 15°C)=4.96; Q₁₀ (10° to 20°C)=1.22. The amount of algae cells ingested per mussel per hour is directly related to food concentration. The maximum number of cells filtered/mussel/h in an algal suspension of 70x10⁶ cells/l was 21.5x10⁵ cells/h. Cell concentrations of up to 40x10⁶ cells/l were swept clear without producing pseudofaeces. The critical cell density for M. edulis was reached at algal concentrations of 70 to 80x10⁶ cells/l. Above these concentrations no normal filtration activity was observed.     

Role of the kidney in determining the whole soft tissue zinc concentration of individual mussels (Mytilus edulis)

Mussels (Mytilus edulis) were collected from an unpolluted site in Newfoundland in 1984 and 1985 in order to determine the zinc levels of the whole soft tissues and individual organs. Individuals were collected in such a way as to eliminate all known physiological and environmental factors which might have led to differences in zinc concentration between individual mussels. The inherent variability of the whole soft tissue zinc concentration was found to be very high with a coefficient of variation (CV) around 40%. Most individual organs had relatively low mean zinc concentrations with small values for CV (10–18%). However, the kidney showed a relatively high mean zinc concentration (828 ppm) with a remarkable degree of variability (94–3 410 ppm) (CV=78%) and is responsible not only for the high degree of inherent variability in the population but also for the high levels of zinc found in the whole soft tissue of some individual mussels. Those individuals with high whole soft tissue zinc concentrations often had high kidney zinc concentrations and stored as much as 53% of their zinc load in the kidney (which accounted for only 1–3% of the total weight). However, other individual mussels with low whole soft tissue zinc concentrations had low kidney zinc concentrations and stored as little as 3% of their body zinc load in the kidney. Hence, the role that the kidney plays in zinc storage varies markedly from mussel to mussel.M.S.R.L. Contribution No. 624     

Growth and size structure in a baltic Mytilus edulis population

Since Mytilus edulis L. has very few predators and competitors for space, it has become a biomass dominant in the Baltic proper covering hard substrates from the water surface to more than 30 m depth. In order to investigate the factors controlling size and production in a Baltic M. edulis population, growth was studied by the analysis of annual growth rings, measurements of caged individuals and the analysis of size classes in the population, and on settlement ropes. The total number of mussels in a representative mussel bed at 4 m depth varied between 36 000 and 158 000 ind · m^-2 during the year, mainly due to variations in very small mussels (<2 mm), whereas the abundance of mussels 2mm was rather constant between about 17 000 and 28 000 ind · m^-2. Maximum numbes of mussels < 2 mm, amounting to 132 000 ind · m^-2, were found after settlement in summer, but still half a year later in spring, 65 000 ind · m^-2 < 2 mm were registered, due to very strong intraspecific competition for food and space leading to the competitive suppression of small individuals and large variations in growth rates. Due to the special size-structure of the population only the analysis of annual growth rings could be used to estimate natural shell growth. From being very low in the smallest mussels, growth was linear between about 2–10 yr of age, corresponding to about 3–20 mm length, after which it decreased with a L=32 mm. Over the linear interval, growth in the populations from 3–6 m and 10–15m depth was 3.1 and 2.2 mm · yr^-1, respectively. Meat growth showed strong annual variations mainly due to gonad production. Starving mussels could, however, while utilizing energy reserves, survive losses of up to 78% of their meat biomass. This ability of M. edulis to respire away its own biomass and its apparent tolerance of weight loss has important implication. It will drastically reduce the energy flow to destruents from mussels dying naturally, which is of special significance in the Baltic, where predators and scavengers are scarce. It enables the mussels to endure bad food conditions and buffer strong seasonal variations in food abundance, maintaining the strongly food-and space-limited Baltic M. edulis population at the carrying capacity of the area.     

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.     

Mytilus trossulus in Northern Europe

From data on allozyme, nuclear DNA and mitochondrial DNA markers, we show that the originally North Pacific/Northwest Atlantic mussel Mytilus trossulus is widespread on North European coasts, earliM. trossuluser thought to be inhabited only by Mytilus edulis. Several local occurrences of , interspersed with a dominant M. edulis, were recorded on the North Sea, Norwegian Sea and Barents Sea coasts of Norway and the Barents and White Sea coasts of Kola Peninsula in Russia. The proportion of M. trossulus genetic background observed at any one site varied from 0 to 95%. These new occurrences are not related to the previously known, introgressed M. trossulus population that occupies the Baltic Sea. The new northern occurrences retain both the F and M M. trossulus mitochondria, which have been lost from the Baltic stock. While hybridization takes place wherever M. trossulus and M. edulis meet, the extent of hybrization varies between the different contact areas. Hybrids are rare, and the hybrid zones are bimodal in the northern areas; more interbreeding has taken place further south in Norway, but even there genotypic disequilibria are higher than those in the steep transition zone between the Baltic mussel and M. edulis: there is no evidence of a collapse toward a hybrid swarm unlike in the Baltic. The Barents and White Sea M. trossulus are genetically slightly closer to the NW Atlantic than NE Pacific populations, while the Baltic mussel has unique features distinguishing it from the others. We postulate that the presence of M. trossulus in Northern Europe is a result of repeated independent inter- or transoceanic cryptic invasions of various ages, up to recent times.     

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.