The genetic structure of Mytilus chilensis (Hupé 1854) populations along the Chilean coast based on RAPDs analysis

The Chilean blue mussel Mytilus chilensis is an important commercial species. However, little has been published on the population genetics of this species, despite the need to implement management and conservation policies. Randomly amplified polymorphic DNA‐polymerase chain reaction analysis was used to estimate genetic variation within and between eight natural populations along the whole range of its Chilean natural distribution (ca. 1900 km from Arauco (VIII Region) to Punta Arenas (XII Region)). The values of Nei's unbiased genetic distance, D (0.030–0.107), among populations were small, despite the large geographic separation. A mantel test using 50 000 randomizations showed evidence for a significant correlation (r=0.74, P<0.05) between genetic and geographic (coastal) distance. Punta Arenas population was the most genetically differentiated from the others, although the scale of differentiation was not large (D=0.076–0.107). The levels of gene flow (Nm=1.55) found in this study prevent differentiation among populations by genetic drift. This is the result of the long‐lived planktotrophic larvae of M. chilensis, which provides this species with considerable dispersal ability throughout its range, which is favoured by the ocean currents along the Chilean coast. A restricted larval dispersal towards the north due to the Cape Horn Current derived from the West Wind Drift could be the cause of the higher genetic differentiation of Punta Arenas population from the northern populations. For management purposes of the M. chilensis fishery, the results provide no evidence for discrete stocks, with the possible exception of the Punta Arenas population. The present study provides the baseline data in order to continue further characterization of these mussel populations, considering the great increase in aquaculture of this species.     

PCR-based nuclear and mtDNA markers and shell morphology as an approach to study the taxonomic status of the Chilean blue mussel,Mytilus chilensis (Bivalvia)

Most recent authors have called the Chilean blue mussel (formerly Mytilus chilensis) Mytilus edulis. Mussels from four nominal species (Mytilus edulis, M. trossulus, M. galloprovincialis and M. chilensis) were collected from the eastern coast of Newfoundland, Canada, New Zealand and the southern coast of Chile. Canonical discriminant analysis of selected shell morphometrical characters showed that although there is some overlap in canonical variates, the three nominal species, M. edulis, M. trossulus and M. chilensis can be morphologically divided into three distinct groups. The two nuclear-DNA markers (ITS and Glu-5) and the mtDNA marker studied indicate that the nuclear and mitochondrial genomes of M. edulis and M. trossulus are different. M. edulis, M. galloprovincialis and M. chilensis share similar restriction fragment patterns for the mtDNA and for the ITS nuclear-DNA marker, while the Glu-5 polymerase chain reaction (PCR) assay showed that the M. edulis and M. chilensis genotypes disagree, producing species-specific banding patterns that can be used as a diagnostic marker between the two nominal species. The molecular and morphometric data reported here give more evidence to support the view of other authors that the Chilean blue mussel should be included as M. edulis subspecies: Mytilus edulis chilensis.     

Heritability of body size in the Chilean blue mussel (Mytilus chilensis Hupé 1854): effects of environment and ageing

Selection, either natural or artificial, induces an inter‐generational response only when the trait exhibits additive genetic variation. An index of additive genetic variation is narrow‐sense heritability (h²): the ratio between additive genetic variance and phenotypic variance. Here, we present narrow‐sense heritabilities for mass and size in the Chilean blue mussel Mytilus chilensis at different ages and in three different populations. We found that h² changed with age and that maternal‐non‐additive variance was high, generating relatively low estimates of h² in most cases. Our results, together with previous studies, suggest that genotype‐by‐environment interaction does not appear to play a significant role. However, we found that non‐additive and common environmental variance are important determinants of phenotypic morphological variation in body size in M. chilensis. Further studies are needed to establish the precise contributions of these sources of variation, to assess the real potential for response to selection.     

Comparison of the physiological energetics between Mytilus chilensis,Mytilus galloprovincialis and their hybrids,under laboratory conditions

Laboratory‐produced juvenile individuals of the species Mytilus chilensis, M. galloprovincialis and their hybrids were subjected to physiological measurements under an experimental diet of Isochrysis galbana (30 × 10⁶ cells L^?1), 13°C temperature and a salinity of 30 psu. Pure species individuals showed a higher clearance rate (CR). Mytilus chilensis had a CR of 1.13 L h^?1, while M. galloprovincialis registered only 0.78 L h^?1. Also, pure taxa registered higher values (above 70%) of absorption efficiency when compared with hybrid individuals. Ammonia excretion in M. chilensis and M. galloprovincialis was 1.5% and 0.4%, respectively, while hybrids registered significantly lower values. Under these experimental conditions, M. chilensis registered the highest scope for growth (P < 0.05), compared with M. galloprovincialis and their hybrids. However, the net growth efficiency index (K ₂) in hybridization type I (♀Mg × ♂Mc) was higher (P < 0.05) than other experimental groups. The invasive mytilid M. galloprovincialis showed values that are very similar to those obtained with the hybridization I group (♀Mg × ♂Mc). Finally, we discuss that water temperature is an important factor in the biogeographic separation of both species and the potential effects that the settlement of the invasive species may have for Chilean mussel production.     

Conservation status of two endangered freshwater mussel species in Bavaria,Germany: Habitat quality,threats, and implications for conservation management

1. The freshwater pearl mussel (Margaritifera margaritifera) and the thick‐shelled river mussel (Unio crassus) are relatively widespread across Europe, but are strongly declining and are now protected by the European Habitats Directive. In the course of this study, 20 pearl mussel and 14 thick‐shelled river mussel streams in Bavaria, Germany, were investigated.
2. The mussel populations were mapped to determine population size and age structure. For the assessment of habitat quality, host fish abundance and physicochemical parameters were investigated, e.g. substratum quality, water chemistry, redox potential, and turbidity. Furthermore, potential risks for the populations such as predation or river maintenance were also recorded and assessed.
3. The average population size and recruitment rates of M. margaritifera populations were lower than in U. crassus populations, with 3517 (2.2% juveniles) compared with 5566 (41.4% juveniles) individuals, respectively. On average, 22.3% of particles were smaller than 0.85 mm in diameter at M. margaritifera sites, whereas the mean proportion of fine particles at U. crassus sites was twice as high, at 41.3%. Other parameters such as redox potential or electric conductivity also indicated more favourable habitat conditions in M. margaritifera streams. Unio crassus seems to be less vulnerable to adverse substratum texture and increased nutrient levels than M. margaritifera.
4. The main threats for U. crassus were physical habitat destruction, predation by muskrat, or a lack of host fish, whereas M. margaritifera mainly suffered from siltation leading to a lack of oxygen supply to the interstitial zone, affecting recruitment. Consequently, conservation strategies need to be species‐specific and address stream‐specific reasons for decline. As a basis, accurate and comparable monitoring data are necessary, which implies the standardization of monitoring protocols.

     

Shell-shape variation along the latitudinal range of the Chilean blue mussel Mytilus chilensis (Hupe 1854)

Shell‐shape variation in the Chilean blue mussel Mytilus chilensis was examined in eight populations covering the totality of its distribution range, which represents over 1800 km of latitudinal gradient in the southeastern Pacific. The shell outline shapes were assessed using elliptic Fourier analysis followed by canonical variates analysis. Fourier coefficients showed a highly significant morphological variation between the populations studied. Canonical variates analysis showed a visual association of the First canonical with convexity of ventral edge and umbo shape, while the second axis was associated with shell elongation. Previously described genetic patterns were marginally congruent with our morphometric pattern, though geographic distance had a stronger effect on morphology. Shape change (elongation) was correlated with latitudinal clinal variation. The morphometrical analysis proved to be an important tool for evaluating the components of shell‐shape variation in mussels and to document association patterns with geographical and ecological variables. Such patterns are useful to establish a wide field of work, including sexing, stock recognition, traceability studies and productive management in general.     

Comparative growth of cultured blue mussels, Mytilus edulis, M. trossulus and their hybrids, in naturally occurring mixed-species stocks

In Atlantic Canada, both Mytilus edulis and M. trossulus occur naturally in sympatric populations and readily hybridize forming genotypically complex assemblages that vary widely in relative species composition throughout the region. We monitored growth in single year‐class cohorts of rope‐cultured Mytilus spp. during a 14‐month period from sleeving of seed to harvest at three commercial mussel farms in Notre Dame Bay, Newfoundland, Canada. Electrophoretic variation at the mannose phosphate isomerase (Mpi) enzyme locus was used to categorize the two species (M. edulis and M. trossulus) and their hybrids. Seed populations varied significantly among sites in relative proportions of each species and hybrids. Growth in shell length, wet weight and shell weight varied significantly with Mpi genotype at all three farm sites. Growth in flesh weight varied significantly with Mpi genotype at two of the three sites. Growth rates for all weight variables were consistently significantly greater in M. edulis than in sympatric M. trossulus. Growth in shell length was significantly different between sympatric M. edulis and M. trossulus at only one farm site. Growth rates of hybrids were typically intermediate between sympatric M. edulis and M. trossulus, but were often statistically similar to one or other of M. edulis or M. trossulus. We conclude that the production of rope‐cultured blue mussels in Atlantic Canada is significantly influenced by genotype‐dependent growth variation in naturally occurring mixed‐species stocks. Our results also suggest that farm site production may be increased and intrasite variability reduced at farm sites currently using indigenous mixed‐species stocks by substitution and growout of transplanted seed from unispecific M. edulis stocks.     

Regional population genetics and global phylogeography of the endangered highly migratory shark Lamna nasus: Implications for fishery management and conservation

1. The porbeagle shark Lamna nasus is a large highly migratory shark distributed in cold and temperate marine waters of the North Atlantic and Southern Hemisphere (SH). According to the International Union for Conservation of Nature, the porbeagle is assessed globally as Vulnerable and regionally as Critically Endangered in the North Atlantic and the Mediterranean Sea. This study explored, for the first time, the population genetics of L. nasus at a regional (south-east Pacific Ocean) and global scale.
2. In this study, the null hypotheses of no genetic discontinuities among populations (i) within the SH, and (ii) between the SH and Northern Hemisphere (NH) were tested. Also, the demographic history of L. nasus in different ocean basins was assessed. Two mitochondrial markers (Control Region [CR] and cox1) well suited for population genetics inferences in sharks were used.
3. Spatial–genetic analyses suggested two genetic clusters co-occurring in the south-eastern Pacific Ocean. A two-way ANOVA using the cox1 but not the CR mtDNA fragment detected an effect of genetic identity on shark body size. Phylogeographic analyses, haplotype networks, and analyses of molecular variance demonstrated genetic differences between populations from the NH and SH but not among populations in the SH.
4. Migration estimates indicated limited current maternal gene flow between the two hemispheres but high gene flow within hemispheres. Two well-defined haplotype groups with star-like shapes inhabited all ocean basins in the SH. These results could reflect a historical scenario of reproductive isolation and more recent mixture among previously isolated populations in the SH. A Bayesian skyline plot analysis indicated sudden population expansion in the SH occurring ~100–125 kya.
5. This study highlights the need for additional studies focusing on the population genomics (using nuclear markers, i.e. single nucleotide polymorphisms) and the general biology of L. nasus to explore the existence of genetically dissimilar populations in the SH. Such studies will help implementing efficient genetic monitoring programmes.

     

Can patch size and patch distance improve the recolonization of mussel‐seed beds exploited for aquaculture?

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Genetic structure of Irish freshwater pearl mussels (Margaritifera margaritifera and Margaritifera durrovensis): Validity of subspecies,roles of host fish,and conservation implications

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The role of geomorphology in substratum patch selection by freshwater mussels in the Hawkesbury–Nepean River (New South Wales) Australia

• 1. Microhabitat preferences of freshwater mussels and associated substrate characteristics were investigated across a range of geomorphic reaches in the Hawkesbury–Nepean River, Australia.
• 2. The structure of substratum patches available was strongly influenced by geomorphic reach type. In each reach type, mussel distribution was most frequently correlated with coarse sand and a roughness element characteristic for that reach. Roughness elements such as boulders and cobbles create a flow refuge and were linked with mussel size.
• 3. Small mussels tended to be associated with boulder‐stabilized habitats and medium sized mussels with cobble habitats. Large mussels rarely co‐occurred with any particular roughness element. Individual species were strongly linked to geomorphic reach type. This association may be due to species' differences in ability to colonize available microhabitat types.
• 4. The highly tolerant Velesunio ambiguus dominated shale reaches, characterized by fine sediments and human impacts. In contrast, Hyridella depressa dominated in gorges, utilizing small flow refuges among boulders, while H. australis were present in low densities across a range of substrate conditions.
• 5. The persistence of multispecies assemblages in mussel beds throughout the Hawkesbury–Nepean River implies similar niche utilization among species. Partitioning of habitats across species on the basis of size suggests some degree of habitat selection, or differential survival. At the local scale, microhabitat characteristics influenced the size distribution and densities of mussel assemblages. Continuing declines in mussel densities are likely to result from ongoing channel modification and increased siltation resulting from changes to riparian vegetation.

Copyright © 2008 John Wiley & Sons, Ltd.     

Demonstrating the practical impact of studies on biotic interactions and adaptation of a threatened unionoid mussel (Margaritifera margaritifera) to its host fish (Salmo trutta)

1. To understand the ecological factors behind the decline of functionally important threatened species with complex life cycles, many different life‐cycle stages need to be investigated. The highly threatened unionoid freshwater mussels, with their complex life cycle, including a parasitic stage on host fish, often have a large influence on biodiversity and ecosystem functioning.
2. The overall aim of the present article is to summarize and discuss the impact of two articles published in Aquatic Conservation: Marine and Freshwater Ecosystems (AQC) on biotic interactions and adaptation of a threatened unionoid mussel (Margaritifera margaritifera) to its host fish (Salmo trutta).
3. The two AQC publications described research on the influence of population size and density of mussels and host fish, and host–parasite interactions between mussels and their host fish, on the recruitment of juvenile mussels.
4. The results from these publications filled gaps in knowledge and resulted in recommendations and incentives for conservation. The results and method development have been used in practical conservation work with threatened mussel species and have been implemented and cited in management handbooks. The outcome of the publications has been implemented in large conservation and restoration projects, and in several recent scientific publications.
5. Specifically, the results from one publication showed that ecological parameters such as mussel and host fish density and population size influenced recruitment of the threatened freshwater pearl mussel. The results from the second publication showed that understanding host–parasite interactions is important for comparing the suitability of host fish strains, and that host fish strains differ in their suitability for mussel infestations. In combination, the articles show that integrating ecological parameters of threatened mussels and their host fish with host–parasite interaction experiments can be an important influence on conservation recommendations, adaptive management and national management programmes for threatened species.

     

Capricious bioinvasions versus uncoordinated management strategies: how the most unlikely invaders can prosper under the current UK legislation framework

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A comparison of growth and biochemical composition of <Emphasis Type="Italic">Mytilus galloprovincialis</Emphasis> (Lmk.) and <Emphasis Type="Italic">Mytilus edulis</Emphasis> (L.) on the West coast of Cotentin,Normandy, France 1999–2000

The blue mussel, Mytilus edulis has been reared along the Normandy coast line since the 1960s. The gonadal cycle of this mussel species shows a sharp decrease in meat quality during the winter period after spawning. This decline in meat quality is so severe that sales have to be suspended from December to July. Another species of mussel, Mytilus galloprovincialis, which is reared in the lagoons along the French shore of the Mediterranean Sea has a different spawning cycle. An experiment was undertaken to study the meat quality of M. Galloprovincialis throughout the year after the spat were transferred from the Mediterranean Sea to the Normandy coast. This species showed an immediate adaptation to the gonadal cycle of M. edulis. Despite suggestions from researchers, no interest was expressed to consider such transfers of M. galloprovincialis in the future.     

Culturing freshwater pearl mussel Margaritifera margaritifera: a breakthrough in the conservation of an endangered species

• 1. The freshwater pearl mussel Margaritifera margaritifera L. is globally endangered and is threatened by commercial exploitation, pollution and habitat loss throughout its range. Captive breeding would be a valuable tool in enhancing the status of M. margaritifera in the UK.
• 2. We have developed a semi‐natural system for successfully infecting juvenile brown trout with glochidial M. margaritifera, and culturing juvenile mussels in experimental tanks where glochidial M. margaritifera can excyst from fish gills and settle into sediment.
• 3. Infected fish had less than 1% mortality. Levels of infection varied among fish. Two yearly cohorts of juvenile M. margaritifera were identified from samples of sediment taken from each experimental tank. Individuals range in size from 1.4 mm (2000 cohort) to >3 mm in length (1999 cohort).
• 4. The number of juvenile M. margaritifera present in the two experimental tanks are estimated to be between 3600 (tank A) and 0 (tank B) for the putative 1999 cohort and between 6000 (tank A) and 13 000 (tank B) for the putative 2000 cohort.
• 5. This pioneering method for large‐scale cultivation of juvenile M. margaritifera is intermediate between the release of infected fish into rivers and the intensive cultivation systems developed in continental Europe and the USA for other species of unionid. This is the first time that large numbers of M. margaritifera have been cultured and represents a significant breakthrough in the conservation of this globally endangered Red Data List species. The method is straightforward and is most cost‐effective when undertaken alongside established hatchery processes.

Copyright © 2006 John Wiley & Sons, Ltd.     

Heritabilities and genetic correlation of shell thickness and shell length growth in a mussel,Mytilus chilensis (Bivalvia:Mytilidae)

Shell thickness in mollusks is generally considered adaptive because of their effects on fitness. However, little is known about the genetic basis of shell thickness. This is important, because the response to selection and the subsequent adaptive microevolution of a trait, such as thickness is only possible when that trait exhibits additive genetic variation. Here, we estimated the narrow‐sense heritability (h²: ratio between additive genetic variance and phenotypic variance) for the traits ‘shell thickness’ and ‘shell length growth’ in a 34‐month‐old cohort of the mussel Mytilus chilensis obtained by using a half‐full sib design and grown in the field. Also, phenotypic and genetic correlations were estimated between both traits. We found that h² showed significant values for shell thickness (0.294 ± 0.194) and length (0.731 ± 0.379). The phenotypic correlation between both traits was positive and significant; however, the genetic correlation between these traits was not. These results suggest both traits can evolve adaptively by selection, but because these traits did not show genetic correlation, it is possible that selection pressure affecting one trait may not affect the other.     

Mytilus species under rope culture in Scotland: implications for management

Mussel (Mytilus spp.) production is one of the most economically important aspects of global aquaculture and, in Scotland, production has increased from 262 t in 1986 to 4,219 t in 2006. Until recently, mussel production in Scotland was considered to be based exclusively on the blue mussel Mytilus edulis, the native species. In Europe, production is known to consist of M. edulis and M. galloprovincialis, while a third less profitable species M. trossulus occurs in the Baltic Sea, where it is unexploited. In Scotland, the sympatric occurrence of M. edulis, M. trossulus, M. galloprovincialis and their hybrids in cultivation in Loch Etive has recently been reported, with significant losses in production attributed to the presence of M. trossulus. Samples of mussels were taken at three depths from 10 rope-farm sites in Loch Etive. The distribution of the Mytilus species and their hybrids in Loch Etive was investigated using the Me 15/16 nuclear DNA locus for species and hybrid identification. All three species and their hybrids were detected and the data were in agreement with the Hardy–Weinberg model suggesting panmixia. Frequencies of M. galloprovincialis and its hybrids were very low. Overall, M. trossulus (37%) was found to be more common than M. edulis (30%) and 23% of the sampled mussels were M. trossulus × M. edulis hybrids. Species distribution did not correlate with year of settlement nor with salinities taken at the time of sampling. There were significant differences in species distribution due to site location and depth, with M. edulis being more frequent at 8 m depth and M. trossulus more common at shallower depths. These differences might be exploitable in management strategies for continuing production, for example to decrease the prevalence of M. trossulus at shellfish farms in favour of the more desirable M. edulis.     

Quantifying the ecological niche overlap between two interacting invasive species: the zebra mussel (Dreissena polymorpha) and the quagga mussel (Dreissena rostriformis bugensis)

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Characterization of single‐nucleotide polymorphism markers in the Mediterranean mussel,Mytilus galloprovincialis

The Mediterranean mussel, Mytilus galloprovincialis, is one of the most important aquaculture species in Europe. Appropriate molecular markers are required to evaluate genetic resources and to trace genealogies in breeding programmes for improving mussel culture. Microsatellites have been commonly applied to this purpose in other species. However, Mediterranean mussel microsatellites have demonstrated high frequencies of null alleles that hamper accurate estimates of population parameters and confident parentage inferences. As alternative markers, we have characterized in silico 25 potential single‐nucleotide polymorphism (SNP) markers in the Mediterranean mussel from expressed sequence tag (EST) public databases. The genotyping of SNPs was performed using a single‐base extension approach. Their polymorphism was evaluated in 47 individuals from an Atlantic population. Out of the 25 potential SNPs tested, 12 were technically feasible (producing a single amplicon) and polymorphic. All were biallelic and had an unbiased heterozygosity ranging from 0.160 to 0.504. One SNP was from a mitochondrial gene. The combined potential of nuclear SNPs for parentage assignment gave an exclusion probability of a false couple of parents of 0.9471. These markers will be useful for evaluating resources and tracing genealogies in genetic breeding programmes implemented to solve the main problems of mussel culture.