Bioinvasion threatens the genetic integrity of native diversity and a natural hybrid zone: smooth‐shelled blue mussels (Mytilus spp.) in the Strait of Magellan

Smooth‐shelled blue mussels of the Mytilus edulis species complex are widely distributed bivalve molluscs whose introductions threaten native marine biodiversity (non‐indigenous species – NIS). The aim of the present study was to identify the species and hybrids of Mytilus present in the Magellan Region (southern Chile). Results indicate that three mussel species of the Mytilus edulis complex are found in the region – M. edulis, M. chilensis (or the Southern Hemisphere lineage of Mytilus galloprovincialis), and M. galloprovincialis of Northern Hemisphere origin. For the first time, alleles of the introduced M. trossulus are reported from the Southern Hemisphere. In the Strait of Magellan the native Pacific blue mussel, Mytilus chilensis and the native Atlantic blue mussel, Mytilus edulis, meet and mix at a natural hybrid zone (about 125 km in length). This is the first record of a natural Mytilus hybrid zone in the Southern Hemisphere and is also the first record of the co‐occurrence of genes from all four Mytilus species in any one region. These results contribute to the knowledge of the biodiversity and delimitation of mussel species in southern South America, and highlight how introduced species may threaten the genetic integrity of native species through hybridization and introgression.     

Introgression between invasive and native blue mussels (genus Mytilus) in the central California hybrid zone

The ecological and genetic factors determining the extent of introgression between species in secondary contact zones remain poorly understood. Here, we investigate the relative importance of isolating barriers and the demographic expansion of invasive Mytilus galloprovincialis on the magnitude and the direction of introgression with the native Mytilus trossulus in a hybrid zone in central California. We use double‐digest restriction‐site‐associated DNA sequencing (ddRADseq) to genotype 1337 randomly selected single nucleotide polymorphisms and accurately distinguish early and advanced generation hybrids for the first time in the central California Mytilus spp. hybrid zone. Weak levels of introgression were observed in both directions but were slightly more prevalent from the native M. trossulus into the invasive M. galloprovincialis. Few early and advanced backcrossed individuals were observed across the hybrid zone confirming the presence of strong barriers to interbreeding. Heterogeneous patterns of admixture across the zone of contact were consistent with the colonization history of M. galloprovincialis with more extensive introgression in northern localities furthest away from the putative site of introduction in southern California. These observations reinforce the importance of dynamic spatial and demographic expansions in determining patterns of introgression between close congeners, even in those with high dispersal potential and well‐developed reproductive barriers. Our results suggest that the threat posed by invasive M. galloprovincialis is more ecological than genetic as it has displaced, and continues to displace the native M. trossulus from much of central and southern California.     

Historical changes in the distributions of invasive and endemic marine invertebrates are contrary to global warming predictions: the effects of decadal climate oscillations

Aim We tested whether a hybrid zone that has formed between an endemic and an invasive species of marine mussel has shifted poleward as expected under a general hypothesis of global warming or has responded instead to decadal climate oscillations. Location We sampled 15 locations on the coast of California, USA, that span the distributions of the two species of marine mussels and their hybrids. Methods Mussels were sampled in 2005–08 and analysed at three nuclear gene loci using methods identical to those used in a study a decade earlier in order to document the genetic architecture of this system. Change in the system was determined by comparing the frequency of species‐specific alleles and multi‐locus genotypes over the intervening decade. Climate variation over the same period was examined by comparing the Pacific Decadal Oscillation (PDO), El Niño/Southern Oscillation (ENSO), upwelling indices and sea surface temperature (SST) during and prior to the study period. Results Contrary to the general expectations of global warming we show that the highly invasive warm‐water mussel Mytilus galloprovincialis and the hybrid zone formed with the endemic species Mytilus trossulus has rapidly contracted southwards. Mytilus galloprovincialis declined in abundance over the northern third of its geographic range (c. 540 km) and has become rare or absent across the northern 200 km of the range it previously colonized during its initial invasion. The distribution of the native species M. trossulus has remained unchanged over the same time period. Main conclusions The large‐scale range shift in the warm‐water invasive species M. galloprovincialis and the hybrid zone it forms with M. trossulus has been exceptionally rapid and is in the opposite direction to that predicted by the global warming hypotheses. This shift, however, is consistent with decadal climate variation associated with the ENSO and the PDO. Since the biogeography of this system was first described in 1999, the PDO has shifted from a warm phase, dominated by frequent and large El Niño events, to a cold‐phase period, with minimal ENSO activity. Thus recent decadal climate variation can oppose global trends in average temperature and this study illustrates the need to integrate the effects of climate change across multiple time‐scales.     

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.     

Genetic diversity of Southern hemisphere blue mussels (Bivalvia: Mytilidae) and the identification of non‐indigenous taxa

The taxonomic and evolutionary affinities of Southern hemisphere smooth‐shelled blue mussels are unclear, with studies using different marker types having identified different relationships among various geographic regions. Using an existing and a new molecular assay, the present study builds on previous work to test the distribution of blue mussels native to and introduced to the Southern hemisphere. Populations of Mytilus were sampled from New Zealand, Australia, and Chile. The nuclear‐DNA marker Me 15/16 was used to identify the taxonomic status of 484 individuals. A new restriction fragment length polymorphism (RFLP) assay was used to identify the hemisphere of origin for a subset of Mytilus galloprovincialis. The Me15/16 marker identified 478 pure M. galloprovincialis from Southern hemisphere sites and six Mytilus edulis/M. galloprovincialis hybrids from the Auckland Islands (New Zealand) and Chile. A cytoplasmic RFLP identified Northern hemisphere M. galloprovincialis in almost every Southern hemisphere region. The presence of native M. galloprovincialis at high latitudes (up to 52°S) has implications for our understanding of environmentally induced selective constraints considered to determine species distributions. Widespread occurrence of invasive Northern hemisphere blue mussels in the Southern hemisphere is documented for the first time. Identification of inter‐specific hybrids (M. edulis × M. galloprovincialis) in Chile and in the Auckland Islands (subantarctic New Zealand) illustrates that environments ranging from international ports to remote protected locations are vulnerable to bioinvasion. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 898–909.     

Influence of genotype and geography on shell shape and morphometric trait variation among North Atlantic blue mussel (Mytilus spp.) populations

The influence of geography and genotype on shell shape (outline) and trait (morphometric) variation among North Atlantic blue mussels and their hybrids has been examined. Shape differences among reference taxa (Mytilus trossulus, Mytilus edulis and Mytilus galloprovincialis) were consistent with an association between taxon‐specific genes and shape genes. Newfoundland M. edulis × M. trossulus populations and northern Quebec M. trossulus populations exhibited an uncoupling of taxon‐specific genes from shape genes, whereas Nova Scotia M. trossulus populations and SW England M. edulis × M. galloprovincialis populations exhibited an association between taxon‐specific genes and shape genes. We found no evidence of a geographic effect (NE versus NW Atlantic) for shape variation, indicating that the genotype effect is stronger than any geographic effect at macrogeographic scales. Pronounced differences were observed in trait variability consistent with an association between taxon‐specific genes and trait genes in European populations, and trait divergence of New York M. edulis from all European mussels. Trait variability in mussels from Newfoundland, Nova Scotia and northern Quebec indicated an uncoupling of taxon genes from trait genes, whereas trait variability in SW England M. edulis × M. galloprovincialis populations was consistent with background genotype, indicating a strong association between taxon genes and trait genes. A pronounced macrogeographic split (NE versus NW Atlantic) regardless of taxonomy was observed, indicating that geography exerts a greater influence than genotype on trait variation at the macrogeographic scale. This is consistent with pronounced within‐taxon genetic divergence, indicative of different selection regimes or more likely of different evolutionary histories of mussels on either side of the North Atlantic. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96 , 875–897.     

Molecular effects on spermatozoa of Mytilus galloprovincialis exposed to hyposaline conditions

Salinity represents a critical environmental and an ecological factor in the reproduction of marine species. As global climate changes and anthropogenic factors affect salinity, in this study, we have analyzed the responses of Mytilus galloprovincialis spermatozoa to hyposaline stress. We exposed mussels, in laboratory tanks, for 24 hr at 18°C to control (35.9 psu) and three hyposaline (17.1, 22.6, and 26.2 psu) conditions, and evaluated the expression of sperm hsp70 and protamine‐like proteins genes. Further we analyzed the electrophoretic pattern, the DNA binding and the release from sperm nuclei of protamine‐like proteins. For all experimental approaches used, the results obtained at 17.1 psu condition were very similar to those obtained in the control condition, while alterations were always recorded at 22.6 and 26.2 psu conditions. Particularly, at 22.6 and 26.2 psu, was observed: 42.5‐ and 17.1‐fold increase in hsp70 expression, respectively, and hypoexpression of PL‐II/PLIV protamine‐like proteins genes. Further, electrophoretic mobility shift assays and salt‐induced release of nuclear proteins from sperm nuclei, revealed alterations in the PL proteins/DNA binding, in these two hyposaline conditions. The similarity between the results obtained in control and in the more severe hyposaline condition (17.1 psu) could indicate a phenomenon of fertility preservation strategy due to gamete plasticity.     

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.     

ASYMMETRIC INTROGRESSION OF MITOCHONDRIAL DNA AMONG EUROPEAN POPULATIONS OF BLUE MUSSELS (MYTILUS SPP.)

Abstract.—Mytilus edulis and M. galloprovincialis are two blue mussel species that coexist in western Europe. Previously, we reported that M. galloprovincialis populations contain female and male haplotypes that are fixed in M. edulis populations as well as unique haplotypes. This study assesses whether paraphyly for these species is due to introgression or incomplete lineage extinction. The lineage extinction hypothesis predicts that the shared mtDNA haplotypes in M. galloprovincialis will be significantly diverged from those in M. edulis and form distinct sequence clades. In contrast, the introgression hypothesis proposes that M. edulis haplotypes have only recently been introduced into M. galloprovincialis through hybridization with relatively little divergence accumulating between the shared RFLP haplotypes. We examined 80 mtl6S gene sequences for both the maternal and paternal mtDNA lineages from mussels sampled from various European populations and found strong support for the introgression hypothesis. In addition, we found that M. edulis mtDNA haplotypes appear to be introgressing into mussel populations in the Baltic Sea, which have predominantly M. trossulus nuclear genotypes, indicating that introgressive hybridization is prevalent among European mussel populations.     

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.     

Byssus attachment strength of two mytilids in mono-specific and mixed-species mussel beds

The mussel Xenostrobus securis is endemic to the brackish waters of New Zealand and Australia, but has successfully invaded the inner Galician Rías of NW Spain, where it coexists with the indigenous mussel Mytilus galloprovincialis. In this laboratory study, the plasticity of the byssus attachment strength of two mytilids was compared by manipulating substratum, salinity, and bed assembly. M. galloprovincialis showed stronger byssus detachment strength than X. securis, despite lower byssus coverage. Both species responded similarly to the substratum, with substantially lower byssus strength on methacrylate, which offered the lowest surface free energy. Byssus detachment values for M. galloprovincialis were lower at lower salinity. In mixed beds, a number of mussels moved upwards, eventually colonising the upper layers of the assemblage. This behaviour increased byssus strength but only for X. securis. X. securis is adapted to a wide spectrum of abiotic conditions, a trait that may promote its dissemination within estuarine environments.     

Facilitation and competition between invasive and indigenous mussels over a gradient of physical stress

The interactions between invasive exotic and indigenous species can have profound harmful effects on the recipient community; however, not all such interactions are negative. Facilitation is increasingly recognised as important in shaping natural communities and is believed to vary under different conditions. Earlier studies have shown that the indigenous intertidal mussel Perna perna initially facilitates survival of the invasive Mytilus galloprovincialis in the low mussel zone by providing protection against waves, but later excludes M. galloprovincialis through interference competition for space. Here, we examined interactions between these species in the mid and upper mussel zones, moving mussels to experimental plots in different combinations of densities and species. Mussels were left on the shore for more than a year and treatment effects on mortality, shell length and condition were compared. In the high zone, treatment had no effects and P. perna showed greater mortality than M. galloprovincialis, indicating that its exclusion from the high shore is due to emersion stress. In the mid zone, treatment had no significant effects on M. galloprovincialis, but multiple comparisons among treatments involving P. perna showed that facilitation occurred. P. perna survived better at higher densities, but survived even better when mixed with the physiologically more tolerant M. galloprovincialis. Length data indicated both inter- and intraspecific competition for P. perna in the mid zone. Whereas facilitation occurs strongly in the low zone (P. perna facilitates M. galloprovincialis) and weakly in the mid zone (M. galloprovincialis facilitates P. perna), the lack of facilitation in the high zone suggests that the probability of facilitation is not linearly linked to increasing physical stress. Instead it is likely to be hump shaped: relatively unimportant under conditions that are benign for a particular species, significant under more severe conditions, and overridden by physical stress under very harsh conditions.     

Mussel and dogwhelk distribution along the north‐west Atlantic coast: testing predictions derived from the abundant‐centre model

Aim We performed the first test of predictions from the abundant‐centre model using north‐west Atlantic coastal organisms. We tested the hypotheses that the density of intertidal mussels (Mytilus edulis and M. trossulus) and dogwhelks (Nucella lapillus) and mussel age and size would peak at an intermediate location along their distribution range. We also assessed the latitudinal variation in critical aerial exposure time. Location North‐west Atlantic coast between Newfoundland (Canada) and New York (USA), covering 1800 km of shoreline. Methods Using a nested design, we measured mussel density, age and size and dogwhelk density in 60 wave‐exposed rocky intertidal sites spread evenly in six regions. Critical aerial exposure times were determined using online data. Results Mytilus edulis peaked in abundance in Maine and was much less abundant in the other regions. Mytilus trossulus peaked in abundance in southern Nova Scotia and Maine, was less abundant in the other regions to the north, and was absent in the southernmost region (New York). Both mussel species were least abundant in a northern region (Cape Breton), although not in the northernmost region (Newfoundland). Critical aerial exposure times were negatively correlated with overall mussel density. Mussel age and size were similar among regions. Dogwhelks peaked in abundance in Maine and were much less abundant in the other regions, being positively correlated with overall mussel density across regions. Main conclusions Density data for M. edulis and N. lapillus provide limited support for an abundant‐centre pattern, while M. trossulus shows a clear ramped‐south distribution. Critical aerial exposure times suggest that physiological stress during summer and winter low tides may be lowest in Maine and southern Nova Scotia, which might partially explain mussel predominance in those regions. Winter ice scour in Cape Breton may explain the abundance trough observed there. Mussel size and age may be more limited by wave exposure at our sites (as they all face open waters) than by regional differences in environmental stress. Dogwhelks, which prey on mussels, seem to respond positively to prey density at the regional scale. Our study supports the notion that, while the abundant‐centre model is a useful starting point for research, it often represents an oversimplification of reality.     

A population genetic study of the hybrid zone of Mytilus trossulus Gould, 1850 and an introduced species,M. galloprovincialis Lamarck, 1819, (Bivalvia: Mytilidae) in peter the great bay in the Sea of Japan

This paper examines the genetic variability of the Pacific mussel Mytilus trossulus and an introduced Atlantic species, M. galloprovincialis, in the northwestern Sea of Japan (Peter the Great Bay and Kievka Bay). The genotyping of individuals from eight populations was carried out using eight polymorphic enzyme loci and two nuclear DNA markers (Me-5 and ITS-1,2); the occurrence frequency of parent species and their hybrids was determined. The enzyme and nuclear markers demonstrated concordant genetic variation. The genotypes of the native species M. trossulus were predominant in the samples studied. The frequency of the introduced species M. galloprovincialis in the total material was relatively low; however, it reached 42 ± 2% in samples that were collected in Possjet Bay near the town of Zarubino in a zone of active international navigation. In this area the greatest number of hybrids was found as well. It is concluded that the invasion of M. galloprovincialis in the northwestern Sea of Japan is continuing; permanent populations of this mussel appeared in Possjet Bay that were not recorded here previously.     

Invasive blue mussels threaten regional scale genetic diversity in mainland and remote offshore locations: the need for baseline data and enhanced protection in the Southern Ocean

Human‐mediated biological transfers of species have substantially modified many ecosystems with profound environmental and economic consequences. However, in many cases, invasion events are very hard to identify because of the absence of an appropriate baseline of information for receiving sites/regions. In this study, use of high‐resolution genetic markers (single nucleotide polymorphisms – SNPs) highlights the threat of introduced Northern Hemisphere blue mussels (Mytilus galloprovincialis) at a regional scale to Southern Hemisphere lineages of blue mussels via hybridization and introgression. Analysis of a multispecies SNP dataset reveals hotspots of invasive Northern Hemisphere blue mussels in some mainland New Zealand locations, as well as the existence of unique native lineages of blue mussels on remote oceanic islands in the Southern Ocean that are now threatened by invasive mussels. Samples collected from an oil rig that has moved between South Africa, Australia, and New Zealand were identified as invasive Northern Hemisphere mussels, revealing the relative ease with which such non‐native species may be moved from region to region. In combination, our results highlight the existence of unique lineages of mussels (and by extension, presumably of other taxa) on remote offshore islands in the Southern Ocean, the need for more baseline data to help identify bioinvasion events, the ongoing threat of hybridization and introgression posed by invasive species, and the need for greater protection of some of the world's last great remote areas.     

Allozyme Loci Selection in the Pacific Mussel Mytilus trossulus under Stressful Conditions

The allozyme variability for 15 loci in two samples of the Pacific mussel Mytilus trossulus collected from a single giant cluster was investigated using the method of gel electrophoresis. One sample was subjected to short-term anaerobic stress and then to a longer aerobic stress, leading to the death of about 85% of the individuals. At some of the loci, significant differences in the genotypic and allele frequencies were found between the samples. The results are suggestive of the differential survival of mussels with different genotypes and alleles at some of the surveyed loci under stress. Our data are in agreement with the hypothesis of the adaptive significance of allozyme polymorphism.