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.     

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.     

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     

A proteomic approach to the study of the marine mussels Mytilus edulis and M. galloprovincialis

In this study, a proteomic approach was applied for the generation of reference maps and subsequently to detect, quantify and compare the global protein expression between two related species of marine mussels, Mytilus edulis and Mytilus galloprovincialis, growing in their own geographical habitats. A comparative study of the protein profiles generated from analytical two-dimensional electrophoresis gels was performed, and changes in protein expression were analysed quantitatively by computer analysis. An average of 1,278 spots per gel was detected in 16 individuals (8 M. edulis and 8 M. galloprovincialis); however, not all spots were included in the study. Expression of 420 spots was compared, and significant differences in the intensity levels were detected in 37 protein spots (8.8%). Fifteen proteins showed higher expression in M. edulis, and 22 proteins, in M. galloprovincialis. The technique of peptide mass fingerprinting using MALDI-TOF (matrix-assisted laser desorption ionisation/time-of-flight) and/or nanoelectrospray double subfragmentation mass spectrometry enabled the unambiguous identification of 15 of these 37 differentially expressed proteins. Most of the identified proteins can be grouped basically into four broad functional classes: cytoskeletal and myofibrillar proteins, proteins associated with stress response, proteins associated with the storage or production of energy, and proteins related to rearrangement in the synthesis of native structures. These results expand our understanding of the molecular differentiation of the two mussel taxa and serve as a useful base for future ecological, physiological and genetic studies.     

Proliferation kinetics of mussel (Mytilus galloprovincialis) gill cells

A technique of sister chromatid differentiation (SCD) using bromodeoxyuridine (BrdU) incorporation and a modification of the fluorescence plus Giemsa (FPG) method was employed to determine cell-proliferation kinetics in gill tissue of the mussel Mytilus galloprovincialis. Dose-dependent proliferation inhibition was examined. In vivo administration of BrdU for 12, 24, 36, 48, 60, 72, 84, and 96 h was studied. Our data show that the highest yield of second-generation metaphase plates is obtained after 60 h BrdU treatment; the duration of a cell cycle is 24 to 30 h. On the basis of these data, a BrdU incorporation period of 48 to 60 h would seem to be most appropriate for the sister chromatid exchange (SCE) tests carried out in gill cells of M. galloprovincialis, whereas the 12 to 24 h exposure would give the best results for replication band analysis.     

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.     

Distribution of Mytilus edulis, M. galloprovincialis, and their hybrids in open-coast populations of mussels in southwestern England

The distribution of Mytilus edulis Linnaeus, M. galloprovincialis Lamarck, and their hybrids was examined in mussel populations in southwest England in 1996 and 1998. This is a region where both parental taxa and populations containing large numbers of hybrids co-occur yet a fine-scale mapping of the hybrid populations has not been conducted. In this study the geographic distribution of hybrid populations was determined in southwest England over 360 km of coast from Tintagel Castle in north Cornwall to Beer in south Devon. Sample localities were spaced from 5 to 20 km apart. Genotypes for individual mussels were determined using PCR to amplify a size polymorphism at the Glu-5' locus that is completely differentiated between M. edulis and M. galloprovincialis. Hybrid populations, characterized by high frequencies of individuals with heterozygous genotypes and a pattern of decreasing frequency of M. edulis alleles with increasing shell length, were continuously distributed along 180 km of open coast in southwest England. This "hybrid patch" was bordered at one end by geographically extensive populations of pure M. edulis and at the other end by nearly pure populations of M. galloprovincialis. Strong natural selection in hybrid populations results in a decline in the frequency of M. edulis alleles with increasing size. Wave-exposure has previously been implicated as the agent producing this pattern of selection, but in the present study the relationship between allele frequency and body size was not correlated with variation in wave shock intensity among localities within the hybrid zone. The transition between hybrid populations and those containing pure populations of M. edulis or M. galloprovincialis is abrupt which suggests that coastal circulation patterns may provide strong barriers to larval dispersal which accounts for the position and maintenance of the hybrid zone.     

Patterns of mtDNA diversity in North Atlantic populations of the mussel Mytilus edulis

Patterns of (female) mitochondrial DNA diversity were investigated in the blue mussel Mytilus edulis. Mytilus edulis is a ubiquitous member of contemporary North Atlantic hard-substrate communities and well represented in studies of this region. Mytilus edulis was surveyed in North America and Europe, as well as mid-Atlantic sites in Greenland, Iceland, and the Faroe Islands. Mitochondrial DNA sequences revealed considerable population structure but no monophyly of haplotypes between any major regions. Coalescent analyses suggest that migration across the Atlantic Ocean has prominently been from North American source populations and that Greenland was colonized recently and exclusively from North America. In North America, there was support for two regional groups along the North American coastline. Surprisingly, we also found evidence of recombination between some Mytilus edulis and Mytilus galloprovincialis female mtDNA sequences, particularly in northern Europe. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. An erratum to this article can be found at     

Physiological energetics of cultivated mussel (Mytilus edulis) populations in two Scottish west coast sea lochs

Physiological responses of suspended cultured mussels, Mytilus edulis L., in two Scottish sea lochs (Lochs Etive and Leven) were investigated in their native and transplanted environments, after 15 d, 4.5 mo and 1 yr acclimatization, during the main growing season of May to September 1992. These measurements were integrated by means of the balanced energy equation, and scope for growth was calculated to assess the performance of each stock. Transplanted mussels showed clear signs of stress during the first 15 d after transfer, with low clearance rates and energy retention and high rates of respiration and nitrogen excretion. There were significant differences in some of the physiological responses (clearance, respiration and excretion rates), scope for growth and growth efficiency between the native populations, with the responses of mussels in Loch Etive being more favourable than those in Loch Leven. With newly transplanted mussels after 15 d acclimatization, almost all these measurements also differed significantly from native mussels in their host site and the original stocks but, with the exception of ammonia excretion rates, all the variables of cross-transplanted mussels after 4.5 mo acclimatization were the same as those of the native stock at the host site. Good agreement between observed long-term growth rates and estimated scope for growth suggests that, like growth rate, differences in physiological responses are mainly controlled by environmental factors; i.e., stock or origin had no significant influence on variations in physiological response (except ammonia excretion). The results further indicate that scope for growth estimated during the main growing season can be used to assess the actual growth rate and to compare sites for on-growing.     

Labial palps of the blue mussel Mytilus edulis (Bivalvia: Mytilidae)

As a basis for understanding the functions of labial palps in the blue mussel Mytilus edulis, the structure and histology of palps were studied using light and scanning electron microscopy. Mussels used in the present study were collected in August 1993 and April 1994. The palp ridged surface is characterized by the presence of a smooth but densely ciliated dorsal fold, upon which rests the corresponding demibranch ventral region. The underside of the dorsal fold and the palp ridges fuse to form vestigial ciliated tracts. The dorsal fold is capable of contraction, allowing it to cover variable amounts of the ridged surface. Two different types of mucocyte are present on the palp ridged surface: subepithelial, glandular, acid-dominant secretion mucocytes and epithelial mucocytes characterized by neutral secretions. In histological section, these mucocytes appear to be concentrated on anatomical features known to intervene in particle handling. The anatomical and histological features of the smooth surface are typical of bivalve labial palps, except that the dense ciliation of the dorsal fold begins in the dorsal region of the smooth surface, indicating the possible origin of this feature. Previous studies on M. edulis point to the palps as the probable site for both ingestion volume control and particle selection; the anatomical basis of the present study should facilitate further research on these aspects.     

Metabolic fate of aromatic amines in the mussel Mytilus galloprovincialis

The postmitochondrial fraction of the digestive gland from the marine mussel Mytilus galloprovincialis possesses FAD-containing monooxygenase (EC 1.14.38) but lacks cytochrome P-450 dependent benzo(a)pyrene monooxygenase (EC 11.4.14.1). This is also evidenced by the ability of the mussel preparation to activate carcinogenic aromatic amines, but not carcinogenic benzo(a)pyrene, to Salmonella typhimurium TA 98 mutagens. This metabolic activity is NADPH dependent. Mussel digestive gland postmitochondrial fraction also possesses the enzymes needed for the detoxicating part of the aromatic amine metabolism: UDP-glucuronyl transferase (EC 2.4.1.17) and -glucuronidase (EC 3.2.1.31). Under the experimental conditions used here, this aromatic amine metabolic pathway converts up to 8% of 2-acetylamino(9-¹⁴C)fluorene, but not (G-³H)benzo(a)pyrene, to water soluble glucuronides. Glucuronic acid stimulates the formation of these glucuronides. The metabolites liberated from these glucuronides by the -glucuronidase treatment could be converted to TA 98 strain mutagens by the carp liver postmitochondrial fraction, but not by the mussel's digestive gland preparation. The presence of such a selective potential for the bioactivation and detoxication of aromatic amines, and not polycyclic aromatic hydrocarbons, in the marine invertebrate(s) may bring new insight to our understanding of the effects and the fate of carcinogens in the marine environment.     

Seasonal variation in mineralization rates (C-N-P-Si) of mussel Mytilus edulis biodeposits

To determine seasonal variability in mineralization dynamics of mussel biodeposits, we applied a multiple-element approach measuring mineralization rates of carbon (C), nitrogen (N), phosphorus (P) and silicate (Si) during three periods (March, August and November). The results of this study showed that mineralization rates vary between seasons and between elements and that mineralization dynamics were influenced by both temperature and biodeposit nutrient composition. Mineralization rates were 3.2 ± 0.4 mmol C, 0.17 ± 0.04 mmol N, 0.06 ± 0.02 mmol P and 3.91 ± 3.75 mmol Si per gram biodeposit (DW) per day, which represented 24 % of the particulate organic C and 17 % of the particulate organic N in mussel biodeposits. Seasonal variability was largest for Si mineralization with 60–80-fold higher rates measured in March compared to August and November. This difference is most likely related to the difference in biodeposit nutrient composition. It was furthermore shown that the labile fraction of biodeposits became mineralized after, respectively, 18, 9 and 13 days during the experimental periods in March, August and November. This indicates that temperature enhances biodeposit decomposition with approximately 2–3 times faster turnover at a 10 °C temperature interval (Q ₁₀ ).     

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.     

Mechanisms of habitat segregation between an invasive (Mytilus galloprovincialis) and an indigenous (Perna perna) mussel: adult growth and mortality

The invasive mussel Mytilus galloprovincialis and the indigenous mussel Perna perna coexist intertidally on the south coast of South Africa through partial vertical habitat segregation: M. galloprovincialis dominates the upper shore and P. perna the lower shore. Recruitment patterns can explain the zonation of P. perna, but not the invasive species. We examined the role of post-recruitment interactions by measuring spatial and temporal differences in adult growth and mortality rates of the two species. Specifically, we tested the hypothesis that interspecific differences in growth and mortality reflect adult distribution patterns. The two study locations, Plettenberg Bay and Tsitsikamma, are 70 km apart with two sites (separated by 300–400 m) per location, each divided into three vertical zones. Growth was measured seasonally using different marking methods in 2001 and 2003. Cumulative adult mortality was measured through summer in 2003/2004. Both species generally grew more slowly upshore, but they showed different effects of season. For P. perna, growth was significantly reduced in winter in the low zone, but unaffected by season in the high zone. For M. galloprovincialis, growth was either unaffected by season or increased in winter, even in the high zone. Thus, growth of P. perna and M. galloprovincialis was reduced under cool winter and warm summer temperatures, respectively; and while growth was more similar between species in summer, M. galloprovincialis grew much faster than P. perna in winter. Mortality of P. perna increased upshore. For M. galloprovincialis, mortality was not zone-dependent and was significantly greater than for P. perna on the low-shore and (generally) across the shore in Tsitsikamma. Both species had higher growth and mortality rates in Plettenberg Bay than in Tsitsikamma. Thus, P. perna seems able to maintain spatial dominance on the low-shore and at certain sites because of higher mortality of M. galloprovincialis. We conclude that seasonality in growth of the two species reflects their biogeographic affinities and that coexistence is possible through pre-recruitment effects that limit the vertical distribution of P. perna and post-recruitment effects that limit M. galloprovincialis.     

Growth,mortality and shell morphology of cultivated mussel (Mytilus edulis) stocks cross-planted between two Scottish sea lochs

One-year-old rope-grown blue mussels (Mytilus edulis L.) were held in experimental lantern nets and cross-transplanted between two sea lochs (Lochs Etive and Leven) on the west coast of Scotland. Growth and mortality were monitored from May 1991 to May 1992, as well as shell morphology in native and transplanted mussels. There were highly significant differences (P0.001) in growth rates between lochs; both native and transplanted mussels performed better in all shell and tissue growth-parameters in Loch Etive than in Loch Leven. Stock also had a significant (P0.05) influence on shell length and live weight, but its contribution to total variance was considerable only in the latter case. Wet and ash-free dry meat weights were governed mainly by site and to some extent by site x stock interaction. Mortality rates were quite low (6 to 7% yr^-1) and did not show any significant variation either between sites or stocks. There were significant morphological differences (ratios of shell length, height and width) between the Loch Etive and Loch Leven populations (P0.001) which persisted for one year after transfer. Stock appeared to be the main factor affecting shell morphology, as opposed to site. Linear regressions of shell height and width on length were significantly different between the native stocks, but became indistinguishable from those of the recipient populations one year after reciprocal transfer. This did not, however, conceal the effect of stock origin on dimensional ratios. It is concluded that site differences related to environmental factors, notably chlorophyll a levels and currents, and also possible stress caused by high levels of Zn and Cu, are major determinants of growth, which are of main interest to aquaculture, while morphological differences most probably result from genotypic variation.     

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     

Introgression and mitochondrial DNA heteroplasmy in the Baltic populations of mussels Mytilus trossulus and M. edulis

A strong clinal change in salinity occurs between the Baltic Sea and the North Sea, Atlantic Ocean, in the Danish Straits, where hybridization zone between mussels Mytilus edulis and M. trossulus has been reported. Eleven samples of mussels were studied from the Danish Straits and the inner Baltic Sea. Extensive introgression of M. edulis alleles from the North Sea into populations throughout the Baltic was ascertained for mitochondrial DNA (mtDNA) and two nuclear markers (ME15–16 and ITS). In the opposite direction, introgression of M. trossulus alleles into the M. edulis background was observed at the EFbis nuclear marker in populations from Kattegat (Danish Straits). While only M. edulis F (female) mtDNA was present in the Baltic, there were still strong differences in frequencies in the control region length variants between the Danish Straits and the inner Baltic samples, and weaker variation in coding region ND2–COIII haplotype frequencies. In the assays of the two mtDNA regions, various patterns of heteroplasmy were detected in 32% of all the studied individual mussels; this includes the presence of distinct, independently inherited M and F mitochondria in males, as well as the presence of two different distinguishable F genomes. The male-inherited M mtDNA genomes are quite common in the mussels from the Danish Straits, but very rare in males from the inner Baltic. Instead, a recombined control region variant (1r), which seems to have taken over the role of the M genome, was present in a number of specimens in the Baltic. Observations of heteroplasmy for two F genomes in some females and males confirm disruptions of the doubly uniparental inheritance mechanism in the hybrid Baltic Mytilus.