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.     

Ecological gradients and relative abundance of native (Mytilus trossulus) and invasive (Mytilus galloprovincialis) blue mussels in the California hybrid zone

Marine communities are experiencing unprecedented rates of species homogenization due to the increasing success of invasive species, but little is known about the mechanisms that allow a species to invade and persist in a new habitat. In central California, native (Mytilus trossulus Gould 1850) and invasive (Mytilus galloprovincialis Lamarck 1819) blue mussels and their hybrids co-exist, providing an opportunity to analyze the mechanisms that determine the distributions of these taxa. Spatial and temporal variation in temperature and salinity and the relative frequencies of these mussel taxa were examined between 2000 and 2004 at four sites in San Francisco Bay and four in Monterey Bay, which were chosen for their different positions along inferred estuarine/oceanic gradients in the hybrid zone. Mussels were genetically identified as the parent species or hybrids by amplifying regions of two species-specific loci: the adhesive byssal thread protein (Glu-5′) and the internal transcribed spacer region of ribosomal DNA (ITS 1). The proportion of M. trossulus at the eight hybrid zone sites correlated negatively with average salinity (R ²=0.60) and positively with maximal temperature (R ²≥0.72), a somewhat unexpected result given what is known about the phylogeography of this species. The proportion of M. galloprovincialis showed the opposite pattern. The proportion of hybrids was correlated neither with habitat temperature nor salinity. Genotypes of mussel populations at an additional 13 sites from Coos Bay, Oregon (latitude 43.35°N) to Long Beach, California (latitude 33.72°N), sampled at various intervals between 2000 and 2004, were also determined. This survey confirmed previous reports that the hybrid zone lies between Monterey and the Cape Mendocino region (latitudes 36.63°N–40.5°N). Within Monterey and San Francisco Bays, however, the temporal comparisons (1990s vs. 2000s) revealed abrupt changes in the proportions of the two parent species and their hybrids on annual and decadal time scales. These changes indicate that the blue mussel populations are in a highly dynamic state. The survey also showed that, regardless of habitat, M. trossulus is consistently of smaller average size than either M. galloprovincialis or hybrids.

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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     

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     

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.     

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.     

Spatial and temporal variation in distribution and protein ubiquitination for <Emphasis Type="Italic">Mytilus</Emphasis> congeners in the California hybrid zone

Along the west coast of North America, the invasive mussel Mytilus galloprovincialis and a native congener M. trossulus overlap in range and compete for habitat in an extensive hybrid zone along central California. The two species have been shown to exhibit differential abiotic tolerances in laboratory studies, yet little is known about how such tolerances affect spatial and temporal patterns of geographic distribution, particularly in areas of competition. We examined distributions of the two congeners and their hybrids in neighboring intertidal and subtidal habitats in Bodega Bay, CA over 2 years, and compared shell length and seasonal ubiquitin (Ub) conjugates to estimate protein turnover and physiological stress for the species at each site. The two species were spatially segregated, with M. galloprovincialis dominating the subtidal habitat, and M. trossulus constituting a majority of the intertidal mussel population. Hybrid individuals appeared in low numbers at both sites. For each habitat, there was no statistical difference between shell lengths of M. galloprovincialis and hybrids but M. trossulus mussels were statistically smaller than the other two. In regards to physiological performance, ubiquitin conjugate values showed different seasonal cycles for the two species, suggesting different periods of peak environmental stress. The highest levels of Ub-conjugated proteins were observed in winter for M. galloprovincialis and in summer for M. trossulus, consistent with the respective range edges for their distributions since Bodega Bay is near the northern range edge of the invader and the southern edge of the native species. These findings suggest that future assessments of Mytilus populations along the California coast may need to consider vertical distributions and seasonal cycles as part of monitoring and research activities.     

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     

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.     

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.     

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.     

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.     

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.     

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     

Tidal height,rather than habitat selection for conspecifics,controls settlement in mussels

Settlement is a major determinant of intertidal populations. However, the energy costs of lost larvae are very high. Accordingly, arrival and attachment on suitable substrata are essential requirements for species’ survival. On the intertidal, the presence of cues left by adult or juvenile conspecifics could be vital for the successful establishment of larvae arriving on the shore. Two mussel species, the indigenous Perna perna and the invasive Mytilus galloprovincialis, co-occur on the lower eulittoral zone on the south coast of South Africa. P. perna dominates the low and M. galloprovincialis the high mussel zones, with co-existence in the mid mussel zone. This study tested the hypothesis of settlement selectivity for conspecifics in these two mussel species, to understand whether the final adult distribution of mussels on the shores is determined by active behavioural and chemical mechanisms. Preferential selection by larvae for conspecifics was tested in the field during the peak settlement period in 2004 in natural mussel beds across zones and through manipulative experiments in the mid-zone where the species co-exist. On natural beds, settlement was determined by counts of settlers attached over 48 h onto artificial collectors. Collectors were placed on beds of P. perna and M. galloprovincialis present at both high- and low-adult densities, as well as in mixed beds. On such natural beds, settlers of both species consistently favored low-zone P. perna beds. Settlement patterns over 24 h onto experimentally created mussel patches consisting of P. perna, M. galloprovincialis or the two species combined beds, set in the mixed zone, did not conform with the results of the natural beds study: settlers of both species settled with no discrimination among different patches. The results indicate that mussels, which are sedentary, lack attraction to conspecifics at settlement. This highlights the importance of tidal height in setting settlement rates, and of post-settlement events in shaping populations of these broadcast spawners.     

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     

Mytilus trossulus in Northern Europe

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

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     

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.     

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.