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.     

Molecular population genetics of male and female mitochondrial genomes in European mussels <Emphasis Type="Italic">Mytilus</Emphasis>

The doubly uniparental system of mitochondrial inheritance (DUI) is best known in marine mussels Mytilus. Under DUI there are two types of mitochondrial DNA (mtDNA). The female type (F) is transmitted to offspring of both genders and the male type (M) exclusively to sons; consequently two distinct mtDNA lineages exist. The M lineage evolves under more relaxed selection than the F lineage resulting in higher polymorphism within the M lineage. Though this polymorphism is expected to make inferences on fine population structure easier using M instead of F data, no comprehensive comparative data exist to support this claim. We sequenced a 1,205 bp fragment of M and F mtDNA comprising parts of the COIII and ND2 genes, and analysed 204 individuals representing three Mytilus species: M. edulis, M. galloprovincialis and M. trossulus from 13 European sampling sites. A clear distinction between Mediterranean and Atlantic populations was found with both M and F data, but much better geographic differentiation was found within the Atlantic using F rather than M data. In particular, Atlantic M. galloprovincialis can be differentiated from Atlantic M. edulis, and further subdivision of Atlantic M. edulis is possible using the F data but not the M data. Multiple tests of selection were carried out to attempt to explain this paradox. We concluded that the overall pattern of polymorphism is consistent with strong purifying selection; not only is this selection relaxed in the M lineage in comparison with the F lineage, but it is also more frequently interrupted by periodic selective sweeps within the M lineage.     

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.     

Distribution of male and female mtDNA lineages in populations of blue mussels,Mytilus trossulus and M. galloprovincialis,along the Pacific coast of North America

Geographic variation in mitochondrial large subunit (16S) ribosomal RNA haplotypes was examined for blue mussels, Mytilus trossulus Gould, 1850 and M. galloprovincialis Lamarck, 1819, sampled from ten sites along the Pacific coast of the USA in January of 1993. Using polymerase chain reaction/restriction fragment length polymorphism (PCR/RFLP) assays we determined haplotype frequencies for both the male and female mussel mitochondrial DNA (mtDNA) lineages. Populations from Morro Bay south to San Diego, California, contained only M. galloprovincialis male and female haplotypes, while those from Arcata Bay, California, north to Port Orford, Oregon, were fixed for M. trossulus haplotypes. Populations from Monterey Bay to Bodega Bay, California, contained a mixture of M. trossulus and M. galloprovincialis haplotypes. Overall only 2 of 97 heteroplasmic individuals had a mixed M. trossulus/M. galloprovincialis mitochondrial genotype indicating that hybridization is uncommon in the populations sampled. Further, there was no evidence of extensive introgression between these mussel taxa at the level of mtDNA. This is in contrast to previously published results which suggested the significant introgression of M. trossulus haplotypes into southern populations containing primarily M. galloprovincialis nuclear genotypes. We feel the discrepancy lies in the ability of our assays to detect haplotypes corresponding to both the male and female mtDNA lineages. Potential explanations for the lack of mtDNA introgression include, low levels of backcrossing between hybrids and parental taxa, epistatic interaction between nuclear and mitochondrial genes and the breakdown of a sex-specific inheritance pattern for mtDNA in hybrids.     

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.     

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.     

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.     

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.     

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.     

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     

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     

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.     

Allozymes and morphometric characters of three species ofMytilus in the Northern and Southern Hemispheres

Many authors have considered the common mussels in temperate waters of the Northern and Southern Hemispheres to be a single cosmopolitan species,Mytilus edulis Linnaeus, 1758. Others have divided these mussels into several subspecies or species. Samples of mussels were collected from 36 locations in the Northern Hemisphere and nine locations in the Southern Hemisphere. Electrophoretic evidence from eight loci indicates that the Northern Hemisphere samples consist of three electrophoretically distinguishable species:M. edulis from eastern North America and western Europe;M. galloprovincialis Lamarck, 1819 from the Mediterranean Sea, western Europe, California, and eastern Asia; andM. trossulus Gould, 1850 from the Baltic Sea, eastern Canada, western North America and the Pacific coast of Siberia. Mussels from Chile, Argentina, the Falkland Islands and the Kerguelen Islands contain alleles characteristic of all three Northern Hemisphere species, but because they are most similar toM. edulis from the Northern Hemisphere, we suggest that they tentatively be included inM. edulis. These South American samples are morphologically intermediate between Northern HemisphereM. edulis andM. trossulus. Mussels from Australia and New Zealand are similar in allele frequency and morphometric characters toM. galloprovincialis from the Northern Hemisphere. FossilMytilus sp. are present in Australia, New Zealand and South America, which suggests that the Southern Hemisphere populations may be native, rather than introduced by humans. Morphometric characters were measured on samples which the allozyme data indicated contained a single species. Canonical variates analysis of the morphometric characters yields functions which distinguish among our samples of the species in the Northern Hemisphere.     

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.     

Mitochondrial DNA heteroplasmy in European populations of the mussel Mytilus trossulus

Mitochondrial DNA (mtDNA) of Mytilus trossulus from the Gulf of Gdansk (southern Baltic) and M. edulis from Swansea Bay, South Wales, UK, collected in 1991, was studied by restriction-enzyme analysis. These two species were more similar to each other in haplotype frequencies than either was to M. galloprovincialis from Britain. M. trossulus resembles M. edulis in having a high frequency of heteroplasmy restricted to males. However, in contrast to M. edulis where restriction-site heteroplasmy predominates, in M. trossulus heteroplasmic individuals possess two genomes which differ in length by up to 3 kilobases.     

Factors influencing differential mortality within a marine mussel (Mytilus spp.) hybrid population in southwestern England: reproductive effort and parasitism

Two species of marine mussel, Mytilus edulis and M. galloprovincialis hybridize on the coasts of western Europe. Studies of hybrid mussel populations have shown that natural selection favors M. galloprovincialis-like genotypes within this hybrid zone. Many hypotheses have been proposed to explain differential mortality in these populations. This study tests two hypotheses addressing factors of mortality in a population, and describes yearly energy storage and reproductive cycles of these two species and their hybrids. No evidence was found that the two taxa have different overall levels of reproductive effort or parasite infestation. They do, however, have asynchronous spawning periods and divergent energy storage strategies. In the year of this study, 1993, the M. edulis genotypic class spawned as a group in June and July. After spawning, they built up a high level of mantle energy-storage tissues that are probably used for gametogenesis in the following winter and spring. The M. galloprovincialis genotypic group, however, spawned asynchronously, beginning in June and finishing by August, and did not build up high levels of energy-storage tissues in summer. These results add a temporal component to the interpretation of selective forces acting to shape this hybrid zone. Vulnerability of each species to mortality factors may differ because of their divergent reproductive and energy-storage cycles. Received: 15 January 1999 / Accepted: 26 July 2000     

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     

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.     

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.     

Molecular dating and biogeography of the neritic krill Nyctiphanes

The genus Nyctiphanes (Malacostraca, Euphausiacea) comprises four neritic species that display antitropical geographic distribution in the Pacific (N. simplex and N. australis) and Atlantic (N. couchii and N. capensis) Oceans. We studied the origin of this distribution applying methods for phylogenetic reconstruction and molecular dating of nodes using a Bayesian MCMC analysis and the DNA sequence information contained in mtDNA 16S rDNA and cytochrome oxidase (COI). We tested hypotheses of vicariance by contrasting the time estimates of cladogenesis with the onset of the major barriers to ocean circulation. It was estimated that Nyctiphanes originated in the Pacific Ocean during the Miocene, with a lower limit of 18 miilion years ago (Mya). An Atlantic–Pacific cladogenic event (95% HPD 3.2–9.6) took place after the closure of the Tethyan Sea, suggesting that dispersal occurred from the Indo-Pacific, most likely via southern Africa. Similarly, the antitropical distribution pattern observed in the eastern Atlantic Ocean likely resulted from recent Pliocene–Pleistocene (95% HPD 1.0–4.97) northward dispersal from the southern hemisphere. Our results imply that dispersal appears to have had a significant role to play in the evolution of this group. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.