First observations of predation by New Zealand Greenshell mussels (Perna canaliculus) on zooplankton

Observations made overseas of predation by blue mussels and zebra mussels on mesozooplankton (>200 μm) have raised concern within New Zealand that the Greenshell mussel, Perna canaliculus, which is cultured in large tonnages throughout hundreds of marine farms within the New Zealand coastal zone, could exert ecologically detrimental effects by preying on zooplankton. We conducted experiments at Clova Bay, Pelorus Sound in May 2002 to determine the rates that P. canaliculus ingests prey, up to and including the mesozooplankton size range. Single mussels from farms were incubated with seawater enriched with zooplankton (>60 μm) in gently circulated 15-l pails. Depletion of chlorophyll-a (chl-a), ciliate microzooplankton, and nauplii, copepodites, and adults of copepods was determined over 5 h, relative to controls with no mussels. Two experiments were made over consecutive days. Gut contents of these experimental mussels, and of mussels examined soon after collection from a farm, were described.Gut contents of experimental and of freshly collected mussels (standard shell length ∼90 mm) had numerous copepod parts, whole copepods and larval bivalves present. Experimental mussels cleared chl-a and ciliates from 59- to 137-l individual⁻¹ day⁻¹, respectively, averaged across the two experiments. Faster ciliate than chl-a clearance was probably caused by the high proportion (56%) of phytoplankton below the retention size for P. canaliculus (ca. 5 μm) and by faster ciliate grazing in controls than treatments. The average clearance rates of adult, copepodite, and naupliar copepod stages by mussels were 20, 31, and 49 l individual⁻¹ day⁻¹, respectively. The clearance rates of each copepod stage were not significantly different between the two experiments. Clearance of nauplii was significantly greater than of adults and copepodites, while adult and copepodite clearance rates were nearly significantly different. The mean lengths of the adult, copepodite, and naupliar copepods were 430, 265, and 165 μm, respectively. The decreasing clearance rates with increasing size and development of prey (from ciliates, through naupliar, copepodite to adult copepods), suggested that prey escape ability, related to body size and/or morphology, affected capture rates. Mussel faecal samples indicated complete digestion of the gut contents. Pseudofaecal samples showed very low rejection rates of mesozooplankton by mussels. The results are considered in context of current biophysical modelling studies of impacts of large mussel farms in New Zealand. Designs of future experiments to improve accuracy of estimates of mesozooplankton clearance rates by P. canaliculus are considered.     

Factors affecting horizontal transmission of the microsporidium Amblyospora albifasciati to its intermediate copepod host Mesocyclops annulatus

Factors that directly impact horizontal transmission of the microsporidium Amblyospora albifasciati to its intermediate copepod host, Mesocyclops annulatus were examined in laboratory bioassays. Results were evaluated in relation to life history strategies that facilitate persistence of the parasite in natural populations of its definitive mosquito host, Ochlerotatusalbifasciatus. A moderately high quantity of meiospores from mosquito larvae was required to infect adult female copepods; the IC50 was estimated at 3.6 × 10⁴ meiospores/ml. Meiospore infectivity following storage at 25 °C was detected up to 30 days, while meiospores stored at 4 °C remained infectious to copepods for 17 months with virtually no decline in infectivity. Uninfected female M. annulatus are long-lived; no appreciable mortality was observed in field-collected individuals for 26 days, with a few individuals surviving up to 70 days. The pathological impact of A. albifasciati infection on M. annulatus resulted in a 30% reduction in survivorship after 7 days followed by gradual progressive mortality with no infected individuals surviving more than 40 days. This moderate level of pathogenicity allows for a steady continual release of spores into the environment where they may be ingested by mosquito larvae. Infected female copepods survived in sediment under conditions of desiccation up to 30 days, thus demonstrating their capacity to function as a link for maintaining A. albifasciati between mosquito generations following periods of desiccation. The susceptibility of late stage copepodid M. annulatus to meiospores of A. albifasciati and subsequent transstadial transmission of infection to adult females was established.     

Isolated and combined impacts of blue mussels (Mytilus edulis) and barnacles (Balanus improvisus) on structure and diversity of a fouling community

Effects of two presumably dominant competitors, the blue mussel Mytilus edulis and the barnacle Balanus improvisus on recruitment, population dynamics and community structure on hard substrata were experimentally investigated in the subtidal Kiel Fjord, Western Baltic. The hypothesis that blue mussels and/or barnacles are local dominants and strongly influence succession and community structure was tested by monitoring succession in the presence and absence of simulated predation on either or both species. Manipulations included blue mussel removal, barnacle removal, combined blue mussel and barnacle removal, as well as a control treatment for natural (non-manipulated) succession. In the second part of the experiment, recovery from the treatments was monitored over 1 year.During the manipulative phase of the experiment, blue mussels had a negative effect on recruitment of species, whereas barnacles had no significant effect. Even so, a negative synergistic effect of blue mussels and barnacles was detected. Calculation of species richness and diversity H′ (Shannon Index) showed a negative synergistic effect of blue mussels and barnacles on community structure. Additionally, diversity H′ was negatively affected by the dominant competitor M. edulis. These effects were also detectable in the ANOSIM-Analysis. The non-manipulative phase of the experiment brought about a drastic loss of diversity and species richness. Blue mussels dominated all four communities. Barnacles were the only other species still being able to coexist with mussels. Effects of simulated predation disappeared fast.Thus, in the absence of predation on blue mussels, M. edulis within a few months dominates available space, and diversity of the benthic community is low. In contrast, when mussel dominance is controlled by specific predators, more species may persist and diversity remains high.     

Experimental test of biodeposition and ammonium excretion from blue mussels (Mytilus edulis) on eelgrass (Zostera marina) performance

Blue mussels and eelgrass have been found to coexist in many locations. However, knowledge of the interactions between these species is limited. Two experiments were conducted in the laboratory, a “Deposit” and an “Epiphyte” experiment. The Deposit experiment examined possible effects of increasing load of blue mussel (Mytilus edulis) biodeposits on sediment biogeochemistry and eelgrass (Zostera marina) performance. Z. marina mesocosms received normal or high loads of mussel biodeposits (Normal and High), while no biodeposits were added to the Control. High dosage had overall negative effects on Z. marina, which was reflected as lower leaf numbers and biomass and accumulation of elemental sulphur in rhizomes. The sediment biogeochemical conditions were altered, as the mussel biodeposits enhanced sulphate reduction rates and increased sulphide concentrations in the porewater, which may result in sulphide invasion and reduced growth of Z. marina.In the Epiphyte experiment effects of mussel excretion, with particular emphasis on ammonium, on the growth of Z. marina and their epiphytes were examined. A thick cover of epiphytes developed on Z. marina growing together with M. edulis, and the relative growth rate was reduced with 20% compared to plants from control without mussels. Overall the experiments showed negative effects on Z. marina growing together with M. edulis, thereby supporting a preceding field study by Vinther et al. [Vinther, H.F., Laursen, J.S., Holmer, M. 2008. Negative effects of blue mussel (Mytilus edulis) presence in eelgrass (Zostera marina) beds in Flensborg fjord, Denmark. Est. Coast Shelf. Sci. 77, 91-103.].     

Toxin content and toxic effects of the dinoflagellate Gyrodinium corsicum (Paulmier) on the ingestion and survival rates of the copepods Acartia grani and Euterpina acutifrons

Short-term experiments showed that ingestion rates of the copepods Acartia grani and Euterpina acutifrons on different concentrations of the dinoflagellate Gyrodinium corsicum were not usually different from those obtained with the non-toxic and similar sized Prorocentrum triestinum. Long-term experiments showed that no A. grani survived for more than 288 h on concentrated cultures (1500 μg C l⁻¹) of G. corsicum compared with survival rates of 86.7% on the non-toxic P. triestinum. In contrast high and similar survival rates were found for E. acutifrons exposed for the same time to similar concentrations of both dinoflagellates. These results demonstrated that G. corsicum produce toxins which have significant adverse effects on the long-term survival rates of some copepods like A. grani but not on other copepods like E. acutifrons. The possibility of PSP-toxin production by G. corsicum was analysed by chromatographic analysis (HPLC-FD) and mouse bioassay of extracts obtained from cultures maintained in exponential growth phase in f/2-Si and L1 culture mediums. These analyses and the bioassay ruled out the presence of PSP toxins in this dinoflagellate. However, mouse tests provided the first evidence for the presence of some type of NSP toxin in G. corsicum. Haemolytic assays also gave the first positive results for the methanol extract of this dinoflagellate species.     

Assessing the potential benthic-pelagic coupling in episodic blue mussel (Mytilus edulis) settlement events within eelgrass (Zostera marina) communities

Coastal marine seagrass ecosystems are important nursery grounds for commercially and recreationally important species, and they serve as key settlement and recruitment sites for other species. We investigated several years (2001-2003) where episodic settlement events of blue mussels (Mytilus edulis) occurred in Barnegat Bay, NJ, USA. Population assessment indicated that blue mussels settled in eelgrass beds (Zostera marina) in late spring with peak densities exceeding 170,000 m⁻². Based on calculated filtration rates of M. edulis, we determined that for at least 53 days in 2001, the density and size distribution of M. edulis were sufficient to filter the water column volume in excess of twice a day, with maximum calculated filtration rates exceeding 8 m³ water m⁻² day⁻¹. While the settlement event in 2001 was very localized, in 2003, the settlement event was considerably more widespread throughout the bay, with maximum settling densities exceeding 175,000 individuals m⁻². Associated with these high densities, maximum calculated filtration rates exceeded 15 m³ water m⁻² day⁻¹. This filtration potential may have impeded the localized development of a brown-tide (Aureococcus anophagefferens) bloom in 2001, which occurred in other regions of the bay, but the widespread settlement event seen in 2003 may have impeded the development of any brown-tide blooms in Barnegat Bay during that summer. The decline in mussel densities throughout the summer may be a result of elevated water temperatures in this back bay, but at one site, the high settlement of M. edulis was followed by a substantial migration (>40 individuals m⁻²) of small sea stars (Asterias forbesii). In 2001, A. forbesii was a significant factor in reducing M. edulis density by the end of the summer at the Barnegat Inlet site and a community level assessment showed significant positive correlations between mussel aggregations and sea star densities (r=0.68-0.73, P<0.001). At this same site in 2003, the sea stars were again present in high densities (26 m⁻²) and were a potential mechanism for mussel decline. In other regions of the bay, sea star densities are very low, but numerous other predatory species exist, including blue crabs (Callinectes sapidus), green crabs (Carcinus maenus), spider crabs (Libinia spp.), and several Xanthid crabs. Given the high mussel densities seen in this study and the considerable predation by sea stars and other benthic predators, the benthic-pelagic coupling which these mussels provide in this system contributes to the high secondary production in these grass beds.     

A comparative analysis of Alexandrium catenella/tamarense blooms in Annaba Bay (Algeria) and Thau lagoon (France); phosphorus limitation as a trigger

Environmental conditions ultimately leading to blooms of the toxic dinoflagellate Alexandrium catenella/tamarense were investigated at two Mediterranean sites (Annaba Bay, Algeria and Thau lagoon, France). Three years were examined in details: 1992 (a pre-Alexandrium period), 2002 (a year with the first bloom in Annaba) and 2010 (a year with a major bloom in Annaba). Most conditions were similar, but ammonium concentrations were much higher in Annaba (up to 100 μM) than in Thau (up to 10 μM). First records of A. catenella/tamarense were in 1995 for Thau and 2002 for Annaba, and coincided with soluble reactive phosphorus (SRP) decreasing below a concentration of about 1 μM. No other environmental variable could be related to those blooms. Thus, it is likely that the large reductions in SRP at both sites led to phosphorus limitation of a certain number of phytoplankton species and favored the development of A. catenella/tamarense.     

Larval settlement can explain the adult distribution of Mytilus californianus Conrad but not of M. galloprovincialis Lamarck or M. trossulus Gould in Moss Landing, central California: Evidence from genetic identification of spat

We investigated the spatial distribution of adult and newly settled mussels (Mytilus galloprovincialis Lamarck, Mytilus trossulus Gould and Mytilus californianus Conrad) on the shore at Moss Landing, California to test the hypothesis that adult distributions are a result of settlement patterns. Adult M. californianus were most abundant on a wave-exposed rocky jetty and adults of Blue mussels (M. trossulus and M. galloprovincialis) were more abundant inside the protected Moss Landing harbor. Using taxon-specific polymerase chain reactions, we monitored recruitment during continuous 1-2 week intervals on fibrous scrubbing pads for 12 months in 2002-2003. All mussel species settled in greatest numbers on the exposed jetty, and Blue mussels settled in greater numbers there than did M. californianus. Because Blue mussels settled abundantly where their adults were rare, post-settlement mortality appeared to be the strongest influence on adult distribution. In contrast, M. californianus settled mostly in their adult habitat.     

Biological interactions in enclosed plankton communities including Alexandrium catenella and copepods: Role of phosphorus

A microcosm approach was used to test whether: a) growth under unbalanced nutrient conditions (varying N:P ratios) affected the susceptibility of a phytoplankton community including the dinoflagellate Alexandrium catenella (a paralytic shellfish toxin producer) to mesozooplankton grazing, and b) the potential effects of unbalanced nutrient conditions were mediated by changes in toxicity of A. catenella or by other mechanisms. The experimental setup consisted of fifteen 30 l microcosms, filled with water from the Barcelona Harbour and subjected to treatments combining nutrient inputs at three different N:P ratios (Redfield N:P ratio or nutrient-balanced, high N:P and low N:P), addition or omission of A. catenella (an estimated initial concentration of 38 A. catenella cells ml^− 1, a value typical for blooms in harbours of the Catalan coast), and selective addition of a cultured population of Acartia grani. P sufficiency had a strong positive effect on the growth of A. grani, both with or without A. catenella addition, presumably due to enhanced food quality of the prey community. The presence of this copepod resulted in lower concentrations of ciliates, A. catenella, and other dinoflagellates, suggesting active grazing by the copepods. No noxious effects of A. catenella on the copepods were detected at the relatively low cell concentrations of that dinoflagellate used in the experiment.     

Observation of a Bonamia sp. infecting the oyster Ostrea stentina in Tunisia, and a consideration of its phylogenetic affinities

The small non-commercial oyster Ostrea stentina co-occurs with commercially important Ostrea edulis in the Mediterranean Sea, yet its disposition with respect to the destructive pathogens Bonamia ostreae and Marteilia refringens is unknown. We began an evaluation of the Bonamia spp. infection status of O. stentina from Hammamet, Tunisia, in June 2007 using polymerase chain reaction diagnostics followed by histology and in situ hybridization. Of 85 O. stentina sampled, nine were PCR-positive for a Bonamia sp. using a Bonamia genus-specific assay; of these nine, one displayed the uninucleate microcells associated with oyster hemocytes characteristic of Bonamia spp. There was no associated pathology. DNA sequencing of the parasite from this one infected individual revealed it to be of a member of the Bonamia exitiosa/Bonamia roughleyi clade, an identification supported by positive in situ hybridization results with probes specific for members of this clade, and by the morphology of the parasite cells: nuclei were central, as in B. exitiosa, not eccentric, as in B. ostreae. There is no basis for identifying the Tunisian parasite as either B. exitiosa or B. roughleyi, however, as these species are genetically indistinguishable. Likewise, there is no basis for identifying any of the other Bonamia spp. with affinities to the B. exitiosa/B. roughleyi clade, from Argentina, Australia, Spain, and the eastern USA, as one or the other of these named species. Though they are clearly distinct from Bonamia perspora and B. ostreae, justification for drawing species boundaries among the primarily austral microcells with affinities to B. exitiosa and B. roughleyi remains elusive.     

Cytochrome oxidase I sequences reveal possible cryptic diversity in the cosmopolitan symbiotic copepod Nesippus orientalis Heller, 1868 (Pandaridae: Siphonostomatoida) on elasmobranch hosts from the KwaZulu-Natal coast of South Africa

Over the past decade, numerous molecular phylogenetic studies uncovered cryptic diversity within the Copepoda, yet very few investigations focused on symbiotic copepods. Here we report mitochondrial DNA cytochrome oxidase I diversity in the cosmopolitan elasmobranch symbiont Nesippus orientalis off the KwaZulu-Natal coast of South Africa. Analysis of partial COI sequences of copepods sampled from a diversity of shark hosts, revealed the presence of two divergent clades. Diversity within the clades does not appear to be structured based on host species, host individual, geographic locality or time of sampling. However, divergence between the two clades seems to be related to host species. Phylogenetic analyses of representatives from the two clades, along with Nesippus spp., Caligus spp. and Lepeophtheirus spp. outgroups, further supports the distinction between the two clades. Future molecular phylogenetic investigations of widespread copepod symbionts most likely will reveal far greater levels of biodiversity than currently recognized.     

Mussel beds in deeper water provide an unusual situation for competitive interactions between the seastars Leptasterias polaris and Asterias vulgaris

The common seastars Leptasterias polaris and Asterias vulgaris show competitive interactions in shallow subtidal communities in the northern Gulf of St. Lawrence, particularly during summer when aggregations of the two seastars forage on mussel beds at 1-2 m in depth. We examined interactions between the two seastars in a different situation, in a mussel bed at 6 m in depth (a rare situation in this region). In the deeper mussel bed, seastars were three times more abundant than in the shallower beds, and the mussels were larger. The deeper bed disappeared rapidly due to the intense predation. Although decreased prey abundance should have favored interference interactions, we did not detect either partitioning of mussels by size or avoidance of A. vulgaris by L. polaris as previously reported when mussels are in short supply in shallower water. The lack of an avoidance behavior by L. polaris, together with the higher proportion of L. polaris than A. vulgaris that were feeding, suggests that in this situation, the dominance of A. vulgaris (observed in shallower water) is attenuated, or that L. polaris may dominate.     

Impact assessment of an invasive flatworm, Convoluta convoluta, in the Southern Gulf of Maine

Convoluta convoluta (Abildgaard 1806) is a small (2-3 mm long) acoellous turbellarian flatworm from Europe that has invaded the Gulf of Maine within the last 5 years. Although it has been reported in densities of up to 19 individuals/cm², its ecological impact remains unknown. In its native habitat, it consumes harpacticoid copepods and primary settling mussels <0.5-mm shell length. This study estimated the impact of C. convoluta on juvenile blue mussel populations (Mytilus edulis Linnaeus 1758) around the Isles of Shoals in the southern Gulf of Maine, USA as well as looking at their distribution in their new habitat. We surveyed worm densities at sites of differing wave exposure over three substrates (hard substrates, bladed algae, filamentous algae) to quantify patterns of worm abundance. We found worms on all substrate types with their highest abundances occurring in areas of maximal sunlight exposure and minimal physical disturbance. We showed a definite pattern of consumption of mussels in the lab and found C. convoluta to consume up to 35% of primary settling mussels in the field, but only under certain conditions. Per capita impact on juvenile mussels was found to vary greatly in correlation with mussel recruitment rate and water temperature, but not with the consumption of harpacticoids. Our results also suggested that per capita interaction strength was reduced by intraspecific density-dependent competition and water temperature. The overall impact of C. convoluta on mussel populations in the southern Gulf of Maine is therefore estimated to be minimal.     

In situ enclosure experiments on the influence of cultured mussels (Perna canaliculus) on phytoplankton at times of high and low ambient nitrogen

The influence of the cultured mussel Perna canaliculus (Gmelin 1791) on the abundance of phytoplankton was investigated in Pelorus Sound, New Zealand. Four in situ enclosure experiments were undertaken, two in summer when ambient nitrogen was low, and two in winter when it was high. Each experiment had four manipulation types: added mussels; added nitrogen; both mussels and nitrogen added; and control (no additions). In summer, there was a significant increase of chlorophyll a in response to added nitrogen, indicating that the phytoplankton were nitrogen-limited. At this time, mussels caused an increase (11-17%) in phytoplankton biomass, possibly by converting particulate nitrogen to ammonium, making the nitrogen available for phytoplankton utilisation. The highest ambient chlorophyll a concentrations coincided with high ambient nitrogen in the winter. At this time, mussel grazing caused a significant decrease (5-14%) in phytoplankton concentration, indicating that within-farm depletion of phytoplankton is most likely to occur in winter. On an annual time scale, the mussels had a stabilising influence on phytoplankton biomass, reducing high ambient levels in winter and slightly increasing low levels in summer.     

The mitochondrial genomes of Amphiascoides atopus and Schizopera knabeni (Harpacticoida: Miraciidae) reveal similarities between the copepod orders Harpacticoida and Poecilostomatoida

Members of subclass Copepoda are abundant, diverse, and—as a result of their variety of ecological roles in marine and freshwater environments—important, but their phylogenetic interrelationships are unclear. Recent studies of arthropods have used gene arrangements in the mitochondrial (mt) genome to infer phylogenies, but for copepods, only seven complete mt genomes have been published. These data revealed several within-order and few among-order similarities. To increase the data available for comparisons, we sequenced the complete mt genome (13,831 base pairs) of Amphiascoides atopus and 10,649 base pairs of the mt genome of Schizopera knabeni (both in the family Miraciidae of the order Harpacticoida). Comparison of our data to those for Tigriopus japonicus (family Harpacticidae, order Harpacticoida) revealed similarities in gene arrangement among these three species that were consistent with those found within and among families of other copepod orders. Comparison of the mt genomes of our species with those known from other copepod orders revealed the arrangement of mt genes of our Harpacticoida species to be more similar to that of Sinergasilus polycolpus (order Poecilostomatoida) than to that of T. japonicus. The similarities between S. polycolpus and our species are the first to be noted across the boundaries of copepod orders and support the possibility that mt-gene arrangement might be used to infer copepod phylogenies. We also found that our two species had extremely truncated transfer RNAs and that gene overlaps occurred much more frequently than has been reported for other copepod mt genomes.     

Development of a real-time PCR assay for detection of Mytilus species specific alleles: Application to a sampling survey in Scotland

Shellfish aquaculture is a growing industry in Scotland, dominated by the production of the mussel Mytilus edulis, the native species. Recently the discovery of Mytilus galloprovincialis and Mytilus trossulus together with M. edulis and all 3 hybrids in cultivation in some Scottish sea lochs led to questions regarding the distribution of mussel species in Scotland. The establishment of an extensive sampling survey, involving the collection of mussels at 34 intertidal sites and 10 marinas around Scotland, motivated the development of a high-throughput method for identification of Mytilus alleles from samples. Three Taqman®-MGB probes and one set of primers were designed, based on the previously described Me 15/16 primers targeting the adhesive protein gene sequence, and samples were screened for the presence of M. edulis, M. galloprovincialis and M. trossulus alleles using real-time PCR. Mytilus edulis alleles were identified in samples from all 44 sites. Mytilus galloprovincialis alleles were found together with M. edulis alleles extensively in northern parts of the west and east coasts. Mytilus trossulus alleles were identified in samples from 6 sites in the west and south-west of Scotland. Because M. trossulus is generally undesirable in cultivation and therefore preventing the geographical spread of this species across Scotland is considered beneficial by the shellfish aquaculture industry, these 6 samples were further analysed for genotype frequencies using conventional PCR. Although distribution of the non-native species M. galloprovincialis and M. trossulus have proven to be more widespread than previously thought, there is no evidence from our study of either M. trossulus or M. galloprovincialis acting as an invasive species in Scotland. The real-time PCR method developed in this study has proven to be a rapid and effective tool for the identification of M. edulis, M. galloprovincialis and M. trossulus alleles from samples and should prove useful in future surveys, ecological or aquaculture management related studies in both unispecific and mixed species areas of these species.     

Babesia bigemina: In vitro multiplication of sporokinetes in Ixodes scapularis (IDE8) cells

This paper describes the in vitro multiplication process of Babesia bigemina sporokinetes in a cell line (IDE8) from Ixodes scapularis ticks. The inoculum was obtained from hemolymph of engorged females of Rhipicephalus (Boophilus) microplus ticks naturally infected with B. bigemina. These ticks had been fed on calves living in a tick endemic farm in Brazil. Microscopic morphological details are shown to describe the development of the parasite in the tick cells; the identity of the parasite was confirmed by a duplex PCR method.     

Three species of Mytilus and their hybrids identified in a Scottish Loch: natives, relicts and invaders?

Three species of the mussel, Mytilus, occur in the North Atlantic region, M. edulis, M. galloprovincialis and M. trossulus, and hybrid zones are present where their distributions overlap. M. edulis is a native species in the UK. M. galloprovincialis originated in the Mediterranean and its distribution extends northwards along the Atlantic seaboard to Scotland. Baltic Sea mussels have a M. trossulus ancestry but are highly introgressed by M. edulis. In recent decades, farming of mussels on long-line rope culture systems has been introduced into Scotland. On farms in Loch Etive, a form of mussel with a fragile shell and a different shape to either M. edulis or M. galloprovincialis has been increasing in frequency over recent years. Samples of fragile shelled, normal strong shelled and intermediate mussel types were sampled from two farms in 2006 and compared with samples of M. edulis, M. galloprovincialis and M. trossulus from other sources where their species identity is well established. Abundance relative to depth, shell strength, condition index and shell morphology were analysed together with 5 allozyme loci and one nuclear DNA genetic marker (Me 15/16). The fragile shelled mussels, and many of those classed as intermediate, were identified as a mixture of M. trossulus and M. trossulus x M. edulis hybrids. This identification was strongly supported by both morphological and genetic data and is the first record of the presence of M. trossulus in UK waters. M. trossulus in Loch Etive are most likely to be a post-glacial relict population restricted to the low salinity area of the Loch that has recently increased in abundance due to commercial mussel growing activity. In addition, individual mussels of all three species and their hybrids were detected amongst Loch Etive mussels. This is the first genetic demonstration of all three species and their hybrids occurring together in one location in the Atlantic region and provides a unique opportunity to study the processes of speciation, divergence, and introgression in the genus Mytilus.