The musselsMytilus galloprovincialis andM. trossulus on the Pacific coast of North America

Most recent authors have called the bay mussels of the Pacific coast of North AmericaMytilus edulis Linnaeus, 1758. Thirteen samples ofedulis-like mussels were collected from California, Oregon, and Alaska, USA, in 1985, 1986 and 1987. Electrophoretic evidence from wight loci indicates that these samples consist of two genetically distinct groups, neither of which is similar toM. edulis from the Atlantic Ocean. Mussels in southern California are very similar toM. galloprovincialis Lamarck, 1819 from the Mediterranean Sea; it is probable thatM. galloprovincialis was introduced accidentally to southern California. Mussels in Oregon and Alaska are similar to those from the Baltic Sea and parts of eastern Canada; the nameM. trossulus Gould, 1850 has priority for this taxon. In central and nothern California,M. galloprovincialis, M. trossulus and their hybrids co-occur. Despite the presence of hybrids betweenM. galloprovincialis andM. trossulus, the genetic integrity which they maintain across large areas of the world warrants their recognition as two distinct species.     

Implications of movement behavior on mussel dislodgement: exogenous selection in a<Emphasis Type="Italic"> Mytilus</Emphasis> spp. hybrid zone

Rocky intertidal habitats often exhibit high levels of environmental heterogeneity, and the ability of organisms to move between microhabitats is likely to have a profound influence on their rates of mortality and overall fitness. Mussels within the Mytilus edulis complex are morphologically very similar, yet at sites where these species hybridize in southwest England, populations repeatedly show evidence of selection against individuals with alleles specific to M. edulis Linnaeus, in favor of those with alleles specific to M. galloprovincialis Lamarck. Differential movement rates of these two species were examined within simulated mussel beds (gravel substrate) in the winter (February) and summer (July) of 2001. M. edulis-like mussels moved more frequently and more quickly to the exterior of gravel beds than did M. galloprovincialis-like mussels. Coupled with measurements of attachment strength in the field conducted in July 2001, we used a wave force model to examine the probability of dislodgement for each species under a range of water velocities. Results suggest that by preferentially moving to the exterior of beds, M. edulis experiences higher dislodgement rates due to exposure to large hydrodynamic forces than do M. galloprovincialis. As a consequence of lower attachment strengths, M. edulis is also predicted to have higher mortality rates than M. galloprovincialis in interior portions of the bed. Thus, differential movement behavior may contribute to the differential genotype-specific mortality rates observed in the Mytilus spp. hybrid zone in southwest England, and is an example of behavior potentially modifying rates of exogenous selection in an intertidal hybrid zone.Communicated by J.P. Grassle, New Brunswick     

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.     

PCR-based detection of mtDNA haplotypes of native and invading mussels on the northeastern Pacific coast: latitudinal pattern of invasion

Frequencies of mitochondrial haplotypes characteristic of native Mytilus trossulus and introduced M. galloprovincialis were determined in populations along the west coast of North America from San Diego, California, to the Aleutian Islands, Alaska. We also identified the haplotypes of mussels cultured from larvae arriving in Coos Bay, Oregon, during 1988–1990 from sites in Japan in the seawater ballast of ocean-going ships. All mussels from ballast-water samples were M. galloprovincialis. We found that sampled populations north of San Francisco Bay, including Coos Bay, were entirely composed of mussels with the M. trossulus haplotype, while haplotypes of both species were present in all sites in and south of San Francisco Bay. The presence of M. trossulus in southern sites is contrary to evidence from allozyme studies, and we suggest that mtDNA introgression from M. trossulus to M. galloprovincialis may explain this discordance. This study demonstrates that, despite continued transport and release, M. galloprovincialis has not become established in northern sites. Failure to invade the north coast of North America may reflect environmental unsuitability for M. galloprovincialis. However, invasion success may be probabilistic, and the continuing release of M. galloprovincialis larvae may foreshadow a future successful invasion.     

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.     

Growth and survival differences among native,introduced and hybrid blue mussels (<Emphasis Type="Italic">Mytilus</Emphasis> spp.): genotype,environment and interaction effects

The Mytilus species complex consists of three closely related mussel species: Mytilus trossulus, Mytilus edulis, and Mytilus galloprovincialis, which are found globally in temperate intertidal waters. Introduction of one or more of these species have occurred world-wide via shipping and aquaculture. Stable hybrid zones have developed in areas where these species have come into contact, making the invasion process complex. On the east coast of Vancouver Island (VI), British Columbia (BC), Canada, the native (M. trossulus) and introduced species (M. edulis and M. galloprovincialis), as well as their hybrid offspring, occur sympatrically. This study used a common environment experiment to quantify growth and survival differences among native, introduced, and introgressed mussels on VI. Mussels were collected from an area of known hybridization and reared in cages from May to August 2006. The cages were deployed at a local site as well as a remote site (approximately 150 km apart), and the mussels were genotyped at two species-specific loci. Growth and survival, as fitness measures, were monitored: native, introduced, and introgressed individuals were compared between and within sites to determine whether growth and survival were independent of site and genotype. Overall, mussels reared at Quadra Island performed better than locally-reared mussels at Ladysmith. Specifically, introgressed mussels reared at Quadra Island performed better than all genotypes reared at Ladysmith, as well as better than native mussels reared at Quadra Island. Differences in survival and growth among the native, introduced and introgressed mussels may serve to explain the complex hybridization patterns and dynamics characteristic of the VI introgression zone.     

Origin of the antitropical distribution pattern in marine mussels (Mytilus spp.): routes and timing of transequatorial migration

Many marine species, including mussels in the Mytilus edulis species group (i.e. M. edulis L., M. galloprovincialis Lamarck, and M. trossulus Gould), have an antitropical distribution pattern, with closely related taxa occurring in high latitudes of the northern and southern hemispheres but being absent from the tropics. We tested four hypotheses to explain the timing and route of transequatorial migration by species with antitropical distributions. These hypotheses yield different predictions for the phylogenetic relationship of southern hemisphere taxa relative to their northern counter-parts. The three Mytilus species were used to test these hypotheses since they exhibit a typical antitropical distribution and representative taxa occur in both the Pacific and Atlantic. Two types of mtDNA lineages were found among populations of mussels collected from the southern hemisphere between 1988 and 1996; over 90% of the mtDNA lineages formed a distinct subclade which, on average, had 1.4% divergence from haplotypes found exclusively in northern Atlantic populations of M. galloprovincialis. These data indicate that southern hemisphere mussels arose from a migration event from the northern hemisphere during the Pleistocene via an Atlantic route. The remainder of the southern hemisphere lineages (<10%) were very closely related to mtDNA haplotypes found in both M. edulis and M. galloprovincialis in the northern hemisphere, suggesting a second, more recent migration to the southern hemisphere. There was no evidence that southern hemisphere mussels arose from Pacific populations of mussels. Received: 8 December 1998 / Accepted: 8 November 1999     

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.     

Calanoid copepod,cladoceran, and rotifer eggs in sea-bottom sediments of northern Californian coastal waters: Identification,occurrence and hatching

In Ireland, mussels on exposed rocky shores constitute an interbreeding mixture of two forms of mussels,Mytilus edulis L. and the Mediterranean musselM. galloprovincialis Lmk. This paper presents an in-depth analysis, carried out between October 1984 and December 1986, of genetic variability at two partially diagnostic loci,Odh andEst-D, in two exposed-shore populations ofMytilus spp. in the west of Ireland. Significant differences at theOdh locus were observed in the genetic composition of adult mussels from different tidal levels. These differences were repeatable whether one was analysing replicate samples at a single point in time, samples collected at different points in time, i.e., in different years, or samples collected from different shores. Mussels recruiting to artificial substrates set out for a period of one month at different tidal levels at one of these sites were also observed to be genetically different; mussels higher up the shore exhibited higher frequencies of those alleles characteristically at high frequency inM. galloprovincialis for both theOdh andEst-D loci. Hence, the genetic differences observed in adult mussels are much more exaggerated in juveniles and are already apparent within the first month of benthic life. Possible reasons for the observed microgeographic differentiation are discussed. It is concluded that the observed genetic differences between mussels at different tidal levels arise either in the pelagic/attachment stage or very shortly after settlement.     

Heterozygosity and growth in transplanted mussels

Growth comparisons were made involving mussels (Mytilus spp.) collected from five different localities in Britain in 1980–1981. Two of the localities, Mumbles, South Wales, and Bude, Southwest England, have pure populations of M. edulis and M. galloprovincialis, respectively. The other three, Whitsand Bay, Southwest England, Croyde Bay, Southwest England and Robin Hood's Bay, Northeast England, have hybrid populations with both M. edulis and M. galloprovincialis ancestry. To make growth comparisons, mussels from different populations were mixed in oyster nets and transplanted to three different localities. After periods of transplantation varying between several months and one year, growth was assessed by measuring increase in shell length or dry body weight. Starch-gel electrophoresis was used to assay variation in the transplanted mussels at three allozyme loci partially diagnostic for M. edulis and M. galloprovincialis. The results provide evidence of small growth differences between populations and between allozyme genotypes within populations. These differences accounted for no more than a few percent of the total variation in growth between mussels. Statistically significant results were obtained, but were frequently found not to be reproducible. There is no clear evidence of a growth difference between M. edulis and M. galloprovincialis. Allozyme heterozygotes appear to have growth rates intermediate between allozyme homozygotes; this study thus fails to provide evidence for overdominance with respect to growth rate.     

Hybridization between the blue mussels Mytilus galloprovincialis and M. trossulus along the Pacific coast of North America: evidence for limited introgression

Two species of blue mussel, Mytilus galloprovincialis and M. trossulus, co-occur and hybridize along the Pacific coast of North America. Using a set of polymerase chain-reaction (PCR)-based genetic markers which diagnostically identify these species, we show that they are sympatric from the Cape Mendocino region to the Monterey Peninsula in northern and central California, USA. Mussels with hybrid genotypes were detected in all populations sampled in the region of sympatry, and the frequency of hybrid genotypes in individual hybrid populations ranged from 13 to 44%. Significant frequencies of first-generation backcross genotypes were detected in two individual hybrid zone populations (Berkeley and Monterey Marina) and in the hybrid zone as a whole, indicating that the potential exists for introgression between M. galloprovincialis and M. trossulus. Despite this potential, we found no evidence of advanced introgression beyond first-generation backcrosses, suggesting that gene flow between M. galloprovincialis and M. trossulus has been quite limited. The frequency of mussels with M. trossulus and hybrid genotypes declined abruptly south of Monterey Peninsula, while the frequency of mussels with M. galloprovincialis and hybrid genotypes declined precipitously north of Cape Mendocino. These abrupt genetic discontinuities indicate that this blue mussel hybrid zone is presently positioned between two prominent coastal features and there is little, if any, export of alleles from the hybrid zone into bordering parental populations. Received: 20 August 1997 / Accepted: 26 October 1998     

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     

The Mytilus edulis species complex in southwest England: effects of hybridization and introgression upon interlocus associations and morphometric variation

Loglinear analysis of electrophoretic data from two hybrid Mytilus edulis x galloprovincialis populations in southwest England revealed non-significant associations between genotypes at four allozyme loci, each of which is partially diagnostic for differences between the two taxa. Significant non-random genotypic associations within the context of the non-significant model involved all four assayed loci equally, consistent with their occurrence in a relatively tight linkage group. Multivariate analyses were used to examine electrophoretic variation from the two hybrid populations, and morphometric variation in the hybrid populations and in four allopatric (two M. edulis and two M. galloprovincialis) populations from western Europe While the number of hybrid mussels is high at both sites (22% at Croyde, 53% at Whitsand) the two taxa have largely maintained the genetic differences which exist between them in allopatry. However, morphological differences between the taxa have been eroded for mussels within the hybrid zone, whereas these differences are quite pronounced for mussels from allopatric populations. It is proposed that each taxon within the genus maintains its genetic identity, despite high dispersal potential, widespread hybridization, and high levels of introgression, as a result of adaptation to different environments. The worlwide occurrence of all four Mytilus hybrid zones at ecotones between recognized biogeographical provinces which are characterized by differences in temperature and salinity is consistent with such an interpretation.     

Identification and validation of novel SNP markers in European populations of marine Mytilus mussels

Mussels of the genus Mytilus are widespread in both northern and southern hemispheres. Mytilus taxa are very important components of marine coastal ecosystems, but are difficult to differentiate morphologically. Sequencing and Sequenom MassARRAY iPLEX genotyping technology was used to identify and verify novel SNP markers in three European taxa of Mytilus: M. edulis, M. galloprovincialis, and M. trossulus. SNPs were localized in coding and non-coding sequences of some functionally important genes. Eight SNPs located in genes of the histone family, hsp70 and p53, were discovered and applied as novel markers for Mytilus taxa on a large European scale. Five of these differentiated the M. trossulus genome, two M. galloprovincialis, and one M. edulis. Other SNPs differentiated populations within taxa. The new SNPs will be a valuable tool for population studies of European Mytilus mussels. The percentage of polymorphic SNPs ranged from 19 to 100 % in 24 samples of mussels studied. Populations from Scotland, Mecklenburg Bight, and Norway had over 90 % polymorphic loci. Most loci were in Hardy–Weinberg equilibrium in all samples except for the one from Scotland. The highest percentages of heterozygotes were observed in the Atlantic (Banyuls and Vigo) and North Sea (Tjarno and Westerschelde) populations. An excess of homozygotes was observed in samples from Scotland, Norway, and the Barents Sea. Correspondence and Structure analysis also demonstrated the great heterogeneity of these three samples.     

A molecular approach to the ecology of a mussel (Mytilus edulis – M. trossulus) hybrid zone on the east coast of Newfoundland, Canada

Mytilus trossulus Gould and M. edulis L. coexist in mixed populations in Atlantic Canadian waters. Although there is evidence that the two species hybridize in natural populations and that hybrids produce progeny through backcrosses, no study of the microgeographic distribution of the two forms and their hybrids has been made. Here we examine subtidal samples of mussels taken in July 1997 from two locations in eastern Newfoundland (Canada) and from wave-exposed and protected environments within each location. Shell lengths ranged from 15 to about 90 mm. Mussels were classified as pure forms or hybrids (F₁, F₂ and from backcrosses) based on four diagnostic markers, two allozyme loci (Mpi and Est-D) and two nuclear PCR-based DNA markers (ITS and Glu-5). In addition, a PCR-based mtDNA marker (COIII) was used to characterize the distribution of mtDNA mitotypes among pure and hybrid individuals. There were differences in the proportions of pure M. edulis and M. trossulus between sites and between environments at one location. M. trossulus was the predominant species at one of the two exposed sites. In all four samples, M. trossulus was also the predominant form among small individuals. The frequency of hybrids was 26% overall and did not differ among samples. Hybrids consisted mostly of backcrosses that were M. trossulus-biased among small mussels and M. edulis-biased among large ones. We conclude that both intrinsic genetic factors and extrinsic environmental factors influence the relative frequency of M. edulis, M. trossulus and their hybrids. Received: 29 June 1998 / Accepted: 4 November 1998     

Mitochondrial DNA lineages in the European populations of mussels (<Emphasis Type="Italic">Mytilus</Emphasis> spp.)

Marine mussels (Mytilus spp.) belong to a group of benthic species crucial to coastal ecosystems in Europe and are important for the cultivation industry. In the present study, the nuclear adhesive protein marker (Me15/16) was used for identification of Mytilus species in coastal areas, on a large geographic scale in Europe. Pure M. edulis populations were found in the White Sea and Iceland. M. edulis, M. trossulus and their hybrids were found in the Baltic Sea and the North Sea (Oosterschelde, The Netherlands). M. galloprovincialis, M. edulis and their hybrids occurred in Ireland. M. galloprovincialis populations were observed in the Sea of Azov (Black Sea), the Mediterranean and Portugal. The mitochondrial (mt) DNA coding-region ND2-COIII was studied by PCR (polymerase chain reaction) and RFLP (restriction fragment length polymorphism) assay methods. The mtDNA control region was studied by PCR. Substantial differentiation in the frequency of female haplotypes among the studied populations in Europe was observed. Despite isolation between the Mytilus taxa on a macro-geographic scale, considerable mitochondrial gene flow occurred between populations, with introgression in hybrid zones on a more local geographic scale in Europe. MtDNA of the Atlantic Iberian (Portugal) population of M. galloprovincialis was more similar to mtDNA in populations of M. galloprovincialis and M. edulis from the Atlantic coasts of the Ireland and M. edulis from the North Sea, than to M. galloprovincialis from the Mediterranean. Lower polymorphism of mtDNA in populations of the Baltic and Azov Sea mussels in comparison with other European populations was observed and can be explained by the recent history of both seas after the Pleistocene glaciation. In the M. galloprovincialis population from the Azov Sea, the presence of the male-inherited (M) genome was demonstrated for the first time by sequencing the control region and was observed at high frequency. Possible influence of mussel culture on geographic distribution of the Mytilus taxa in Europe is discussed.     

Microprofiles of oxygen in epiphyte communities on submerged macrophytes

Mussels (Mytilus edulis) transferred in net bags from clean to chronically mercury polluted water readily accumulated mercury during an exposure period of three months. Growth of the transplanted mussels had a diluting effect on the mercury concentration, but the absolute weight of mercury uptake increased throughout the entire period, though there was a tendency for decreased efficiency of the removal of mercury per liter of water filtered by the mussels. Mussels were also translocated from polluted to clean (laboratory) water to depurate mercury. The biological half-lives of mercury was 293 d for M. edulis from the chronically polluted area in contrast to only 53 d for mussels from a temporary massive mercury polluted area near a chemical deposit. In both cases about 75% of the total mercury in the mussels was inorganic, and it is suggested that both inorganic and organic mercury species were immobilized in mussels from the long-term mercury polluted area, whereas the immobilization capacity was exceeded in the short-term mercury exposed mussels near the chemical deposit. Very slow elimination of mercury was observed in the deposit-feeding bivalve Macoma balthica from the chronically polluted area, and about 6% of the total mercury was methyl-+phenyl-mercury. This is more than three times lower than found in M. edulis from the same collecting site. A pronounced difference in the mercury speciation (i.e., total mercury, total organic mercury, methyl-mercury and phenyl-mercury) in M. edulis from the two mercury polluted areas is thought to reflect the different character of the mercury pollution in the two areas.     

Biochemical genetic variation at a leucine aminopeptidase (LAP) locus in blue (Mytilus galloprovincialis) and greenshell (Perna canaliculus) mussel populations along a salinity gradient

In several marine bivalve species, biochemical genetic variation at a leucine aminopeptidase (LAP) locus is associated with environmental variability, primarily salinity fluctuation. Population genetic variation at a LAP locus was investigated here in two sympatric mussel species (Mytilus galloprovincialis and Perna canaliculus) from three locations along a salinity gradient in Wellington Harbour, New Zealand. The data for M. galloprovincialis and P. canaliculus do not support the hypothesis that the LAP polymorphism in either species is associated with salinity variation among adult mussels. Due to the absence of small mussels among the samples it is not possible to discount the hypothesis that selection acts primarily against juveniles, as it does for M. edulis in Long Island Sound, USA. Wellington Harbour populations of M. galloprovincialis exhibited large and often highly significant heterozygote excesses at the LAP locus, whereas populations of P. canaliculus from the same locations exhibited large and highly significant heterozygote deficiencies. The reason for this inter-specific difference in population structure is unknown. If it is the result of selection, this suggests that selection acts differentially upon the two species, because demographic attributes and reproductive biology are very similar in the two species. For both M. galloprovincialis and P. canaliculus, significant levels of population genetic heterogeneity were recorded among three locations separated by only 8 to 12 km. Neither species exhibited shell length-dependent genetic variation at the LAP locus, suggesting that for these two species the LAP polymorphism is not associated with variation in shell length. Received: 30 December 1996 / Accepted: 6 January 1997     

Differential ingestion of zooplankton by four species of bivalves (Mollusca) in the Mali Ston Bay,Croatia

This study provides information about differences in composition of ingested zooplankton amongst bivalve species coexisting in the same area in a period from May 2009 to December 2010. The study was conducted at the Mali Ston Bay (42°51′ N, 17°40′ E)—the most important bivalve aquaculture area in the eastern Adriatic Sea. Stomach content analysis was performed on cultured species—Ostrea edulis and Mytilus galloprovincialis, and commercially important bivalve species from their natural environment—Modiolus barbatus and Arca noae. Results confirmed carnivory in bivalves, both from natural and cultured populations, but cultured species had higher numbers of zooplankters than those living on the seabed. The most abundant taxa were bivalve larvae, followed by tintinnids, copepods, unidentified eggs and gastropod larvae. Recorded numbers of bivalve larvae in M. galloprovincialis stomach were the highest so far reported and show that mussels impact the availability of natural spat.     

Assessment of natural selection in a hybrid population of mussels: evaluation of exogenous vs endogenous selection models

We examined natural selection within a population of marine mussels, sampled in southwestern England in June 1991, containing a high frequency of hybrids between Mytilus edulis L. and M. galloprovincialis Lmk. This system is particularly tractable for the assessment of natural selection because hybridization is common and individual mussels can be aged, allowing changes in the frequency of hybrid genotypes among age classes to be determined. We show that strong viability selection occurs among hybrid genotypes which results in the virtual elimination of M. edulis–like genotypes from the population over a period of 3 years. Recombinant hybrid genotypes are intermediate in fitness, with M. edulis–like genotypes having a lower survival rate and M. galloprovincialis–like genotypes having a higher survival rate than genotypes of mixed ancestry. Since intermediate fitness for hybrid genotypes is inconsistent with endogenous selection models we conclude that the structure and position of this hybrid zone is probably generated by exogenous selection. This pattern of selection is a recurring feature of this hybrid population and likely occurs elsewhere in the hybrid zone. Selection against M. edulis–like genotypes appears to be offset by extensive immigration of larvae dispersed from pure populations of M. edulis. Received: 14 July 1997 / Accepted: 24 February 1998