Interlineage Mytilus galloprovincialis Lmk. 1819 hybridization yields inconsistent genetic outcomes in the Southern hemisphere

A Southern hemisphere lineage of the blue mussel Mytilus galloprovincialis has been diverging in allopatry from Northern hemisphere conspecifics for 0.84–1.2 million years. Secondary contact between Southern and Northern hemisphere mussels in Chile, New Zealand and Australia provides an opportunity to better understand the extent and consequences of extensive range expansion. Non-native M. galloprovincialis and hybrids, as detected from RFLP assays of nuclear and mitochondrial DNA, are present in all three countries and significant cytonuclear disequilibria exist for native homozygotes in Chile and New Zealand, non-native homozygotes in Chile and non-native heterozygotes in New Zealand. Introductions into Australia are rare events given that no pure non-native mussels were detected. Immigration from one or both taxa into the hybrid zone may underlie disequilibria in New Zealand, whilst gender-directional crossing with limited ongoing hybridization contributes to disequilibria in Chile. Hybridization dynamics do not pose a threat to the Southern lineage in Chile and Australia, but in New Zealand, introgression, continued immigration and slight hybridization gender bias towards non-native maternal parents could lead to the regional extirpation of the native lineage.     

A historical perspective of the genus Mytilus (Bivalvia: Mollusca) in New Zealand: multivariate morphometric analyses of fossil, midden and contemporary blue mussels

The taxonomic status of smooth shelled blue mussels of the genus Mytilus has received considerable attention in the last 25 years. Despite this, the situation in the southern hemisphere remains uncertain and is in need of clarification. Recent work suggests that contemporary New Zealand mussels from two cool/cold temperate locations are M. galloprovincialis. However, the distribution of Mytilus in New Zealand ranges from 35 ° to 52 ° south (～ 1800 km), meaning that large areas of the subtropical/warm temperate north and the subantarctic south remain unsampled, an important consideration when species of this genus exhibit pronounced macrogeographical differences in their distributions which are associated with environmental variables such as water temperature, salinity, wave action and ice cover. This study employed multivariate morphometric analyses of one fossil, 83 valves from middens, and 92 contemporary valves from sites spanning the distributional range of blue mussels to determine a historical and contemporary perspective of the taxonomic status of Mytilus in New Zealand. The findings indicated that all fossil and midden mussels are best regarded as M. galloprovincialis and confirmed that contemporary mussels, with one possible regional exception, are also best regarded as M. galloprovincialis. Contemporary mussels from the Bay of Islands (warm temperate/subtropical) exhibited much greater affinity to M. edulis than they did to M. galloprovincialis, indicating that mussels from this area require detailed genetic examination to determine their taxonomic status. The analyses revealed a significant difference between the fossil/midden mussels and the contemporary mussels, consistent with levels of present day differentiation among intraspecific populations and not thought to reflect any substantive temporal change between mussels of the two groups. The continuous distribution of M. galloprovincialis in New Zealand from the warm north to the subantarctic south indicates that the physiology of this species is adapted to a wide range of water temperature conditions. Therefore, the distribution of this species on a worldwide scale is unlikely to be restricted by its adaptation to warm water alone, as has previously been widely assumed. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82 , 329–344.     

An RFLP assay to determine if Mytilus galloprovincialis Lmk. (Mytilidae; Bivalvia) is of Northern or Southern hemisphere origin

Mytilus galloprovincialis is one of three smooth shelled blue mussel species belonging to the Mytilus edulis species complex. Naturally occurring and introduced populations of M. galloprovincialis are widely distributed throughout many regions of the globe. Mytilus galloprovincialis includes morphologically indistinguishable Northern and Southern hemisphere mtDNA lineages that have been separated for ～1 my. To distinguish recently introduced Northern M. galloprovincialis from resident Southern M. galloprovincialis in New Zealand, we developed a 16s rRNA RFLP assay. We compared RFLP assignments of 178 mussels with those generated from a 16s rRNA sequence-estimated phylogeny. All mussels were correctly assigned by the RFLP to their sequence-based phylogenetic placement. This assay allows the rapid identification of Northern and Southern hemisphere M. galloprovincialis and will provide an important tool for monitoring human mediated introductions of otherwise cryptic lineages.     

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.     

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.     

Molecular species identification of morphologically similar mussel larvae reveals unexpected discrepancy between relative abundance of adults and settlers

Understanding the relative importance of larval supply vs. post-settlement mortality underlies studies of marine invertebrate recruitment, yet is often hampered by researchers' inability to identify species among morphologically similar larvae or early juveniles. In New Zealand, two species of co-occurring intertidal mytilid mussels have morphologically indistinguishable settlers: the blue mussel Mytilus galloprovincialis, which is often numerically dominant in the mid-zone of the rocky intertidal, and the ribbed mussel Aulacomya atra maoriana which is often much less abundant. In this study, we obtained samples of newly settled mussels from 6 sample dates April-May 2005 from the rocky intertidal in Wellington Harbour, New Zealand. We used PCR-RFLP of the cytochrome c oxidase subunit I (COI) mitochondrial gene region to identify settlers to species. Of a total of 224 settlers that could be identified, 64% were identified as Mytilus galloprovincialis and 36% as Aulocomya atra maoriana. The percentage of A. atra maoriana in the samples was unexpectedly high and ranged from 22–50% among the sample dates. This study reinforces the need to quantify larval supply at the species level to understand the relative importance of pre- and post-settlement mortality, and also demonstrates the usefulness of the COI region as a species-specific marker for identifying mussel larvae and juveniles.     

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.     

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.     

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.     

Distribution and abundance of fish in a rocky reef environment at the subantarctic Auckland Islands,New Zealand

Summary The distribution and abundance of fish in a rocky reef environment were investigated at the subantarctic Auckland Islands of New Zealand, in June 1986. Fish were counted in transects and specimens were taken. The diversity and abundance of species of large reef fish was low; a total of eight species were observed. Most fish were benthic carnivores. These findings are similar to studies of reef fish in subantarctic waters of Chile. Large differences in species composition were found among locations at the Auckland Islands. The number of species and their abundance was highest near exposed headlands. Few fish were found on reefs at more sheltered inlets. At most locations the nototheniids Paranotothenia angustata and P. microlepidota ranked first in abundance at deep and shallow sites. There were some species that showed differences in abundance with depth. Highest abundance of Bovichthys variegatus, Latridopsis cilaris, Latris lineata and Pseudolabrus cinctus were in deep water on reefs. Large numbers of juvenile P. microlepidota were found only in shallow water. No depth related patterns were found for P. angustata and Mendosoma lineatum. Specimens, other than those counted in transects, were also collected. There were small fish, 4 species of tripterygiids, 1 gobiescocid and 2 syngnathids. The Auckland Islands had closer zoogeographic affinities with islands of temperate and subantarctic New Zealand than with other landmasses of the westwind drift. Although most fish found at the Auckland Islands are found also in temperature New Zealand, the converse of this pattern was not found. The proportional representation of species and trophic groups differed markedly between these regions.     

Shell morphometrics and growth rate of the invasive bivalve mollusc Mytilus galloprovincialis in the Knysna estuarine embayment,South Africa

Shell morphometrics of the invasive mussel Mytilus galloprovincialis were compared at five sites and growth rate at four sites (in four seasons) in the Knysna estuarine embayment. Mussels from two sites (The Heads, Leisure Isle) where wave action was present had shells significantly lower for any length when compared with other more sheltered sites. There was no significant difference in shell width of mussels for any given length among sites. Mussels from The Heads had thicker shells than other sites, and those from Leisure Isle thicker shells than three other embayment sites where shells did not differ in thickness. Growth rate of mussels at two embayment sites (Thesen’s Wharf and Thesen Islands Marina) was greatest in autumn and summer whereas at The Heads and Leisure Isle there was little seasonal difference in growth rate. Growth rate of mussels at Thesen’s Wharf and Thesen Islands Marina was mainly greater in all seasons when compared with mussels at The Heads and Leisure Isle. The more rapid growth rate of mussels at the sheltered embayment sites might in part explain why M. galloprovincialis now dominates the mid- to lower intertidal on hard substrata in this region of the Knysna estuary.     

Genetics and taxonomy of Chilean smooth-shelled mussels, Mytilus spp. (Bivalvia: Mytilidae)

It has been previously established that native smooth-shelled mussels in southern South America possess close evolutionary affinities with Northern-Hemisphere Mytilus edulis L. 1758 (McDonald et al. (1991) [5]). This result has since been challenged by authors claiming that Chilean mussels should be considered a local subspecies of M. galloprovincialis Lmk. 1819. Moreover, morphological, physiological, ecotoxicological and molecular genetic studies on Chilean smooth-shelled mussels still frequently refer to ‘M. chilensis’ Hupé 1854, even though the previous discovery of alien M. galloprovincialis and considerable heterogeneity in shell morphology among samples collected along the Chilean shores raise concerns that different Mytilus spp. species might have been included under ‘M. chilensis’. Here we reviewed the molecular and morphological data available on smooth-shelled mussels from Chile in an attempt to clarify both their genetic composition and their taxonomic status. Using multivariate analysis on sample × allozyme-frequency matrices, we confirmed the widespread occurrence of the Southern-Hemisphere form of M. edulis along the shores from the North Patagonia region of Chile to the southern tip of the South American continent. The populations sampled in southern central Chile showed some evidence of slight introgression from Southern-Hemisphere M. galloprovincialis. Morphological characterization of a sample from Dichato in southern central Chile was consistent with its previous genetic identification as Mediterranean M. galloprovincialis. The occurrence of Southern-Hemisphere M. galloprovincialis in Punta Arenas at the southern tip of the South American continent was also reported. Southern-Hemisphere M. edulis, including native Chilean smooth-shelled Mytilus, should be assigned subspecific rank and named M. edulis platensis d’Orbigny 1846.     

Commensals of Mytilus galloprovincialis in the Black Sea: Urastoma cyprinae (Turbellaria) and Polydora ciliata (Polychaeta)

The intensity of infection of the mussel Mytilus galloprovincialis Lamarck in the Black Sea by the turbellarian Urastoma cyprinae (Graff), which lives on its gills, was found to be higher in larger hosts, reaching a maximum in mussels of 50–70 mm length. Greater numbers occured in mussels inhabiting a silty bottom than in cultivated mussels suspended above the bottom. Over the period 1982–1987, U. cyprinae was most numerous in winter and especially so in years that were colder. The spionid polychaete Polydora ciliata (Johnston) also infects M. galloprovincialis, burrowing into the shell. Young spionids of up to 1 mm length occured in mussels with a shell length of 35 mm. Numbers of this commensal reached a maximum in mussels of intermediate size.     

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.     

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.     

Hidden diversity in high-latitude Southern Hemisphere environments: Reinstatement of the genus Rama and description of Vandenhoekia gen. nov. (Cladophoraceae,Ulvophyceae, Chlorophyta), two highly variable genera

The continental coasts and remote islands in the high-latitude Southern Hemisphere, including the subantarctic region, are characterized by many endemic species, high abundance of taxa, and intermediate levels of biodiversity. The macroalgal flora of these locations has received relatively little attention. Filamentous green algae are prolific in the intertidal of southern islands, but the taxonomy, distribution, and evolutionary history of these taxa are yet to be fully explored, mostly due to the difficulty of access to some of these locations. In this study, we examined specimens of the order Cladophorales from various locations in the high-latitude Southern Hemisphere including the subantarctic (the Auckland Islands, Bounty Islands, Campbell Island, Macquarie Island, and Kerguelen Islands), as well as mainland New Zealand, the Chatham Islands, Chile, and Tasmania. The analyses of the rDNA sequences of the samples revealed the existence of two new clades in a phylogeny of the Cladophoraceae. One of these clades is described as the novel genus Vandenhoekia gen. nov., which contains three species that are branched or unbranched. The amended genus Rama is reinstated to accommodate the other clade, and contains four species, including the Northern Hemisphere “Cladophora rupestris.” In Rama both branched and unbranched morphologies are found. It is remarkable that gross morphology is not a predictor for generic affiliations in these algae. This study illustrates that much can still be learned about diversity in the Cladophorales and highlights the importance of new collections, especially in novel locations.     

Pleistocene separation of mitochondrial lineages of Mytilus spp. mussels from Northern and Southern Hemispheres and strong genetic differentiation among southern populations

Smooth-shelled mussels, Mytilus spp., have an antitropical distribution. In the Northern Hemisphere, the M. edulis complex of species is composed of three genetically well delineated taxa: M. edulis, M. galloprovincialis and M. trossulus. In the Southern Hemisphere, morphological characters, allozymes and intron length polymorphisms suggest that Mytilus spp. populations from South America and Kerguelen Islands are related to M. edulis and those from Australasia to M. galloprovincialis. On the other hand, a phylogeny of the 16S rDNA mitochondrial locus demonstrates a clear distinctiveness of southern mussels and suggests that they are related to Mediterranean M. galloprovincialis. Here, we analysed the faster-evolving cytochrome oxidase subunit I locus. The divergence between haplotypes of populations from the two hemispheres was confirmed and was found to predate the divergence between haplotypes of northern M. edulis and M. galloprovincialis. In addition, strong genetic structure was detected among the southern samples, revealing three genetic entities that correspond to (1) South America and Kerguelen Island, (2) Tasmania, (3) New Zealand. Using the trans-Arctic interchange as a molecular clock calibration, we estimated the time since divergence of populations from the two hemispheres to be between 0.5 million years (MY) and 1.3 MY (average 0.84 MY). The contrasting patterns observed for the nuclear and the organelle genomes suggested two alternative, complex scenarios: two trans-equatorial migrations and the existence of differential barriers to mitochondrial and nuclear gene flow, or a single trans-equatorial migration and a view of the composition of the nuclear genome biased by taxonomic preconception.     

Variation in scope for growth: a test of food limitation among intertidal mussels

Wellington Harbour supports large populations of the mussels Aulacomya maoriana, Mytilus galloprovincialis and Perna canaliculus that are almost entirely absent from nearby coastal locations in Cook Strait. We calculated scope for growth (SFG) using ambient Cook Strait water over a broad temporal scale and a broad range of seston conditions to determine if negative SFG explains this phenomenon. Although all three mussel species had positive mean SFG values, variation in SFG was high and negative values often occurred: A. maoriana 19.1 J g⁻¹ h⁻¹, 43% of mussels showed negative SFG; M. galloprovincialis 1.26, 52% negative SFG; P. canaliculus 45.6, 27% negative SFG. Negative SFG was most often due to negative absorption efficiency caused by metabolic faecal loss that is characteristic of mussels feeding in environments with low seston quality. From our ecophysiology data we constructed a model to estimate SFG based on physiological responses to the narrow range of seston conditions typical of Cook Strait (Model One), and a model to estimate SFG based on physiological responses of mussels to the broad range of seston conditions typical of Wellington Harbour and Cook Strait (Model Two). We used seston data collected over an 18-month period from sites in Wellington Harbour and Cook Strait to derive 159 estimates of species-specific mussel SFG from both models. Both models produced higher estimates of SFG for mussels in the Harbour compared with those at Cook Strait sites. This was consistent with elevated particulate concentrations in the Harbour than at Cook Strait sites, and in agreement with previous studies. For Cook Strait mussels, both models produced negative estimates of net energy balance for long periods of time (several months), whereas for Harbour mussels negative SFG estimates were generally short in duration. We conclude that our short-term laboratory-based determinations of SFG and our long-term bioenergetics modelling estimates do not conclusively support the hypothesis of food limitation for three coexisting taxa of mussels in the intertidal region of Cook Strait, New Zealand. Handling editor: P. Viaroli     

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.     

Jumping Ship: A Stepping Stone Event Mediating Transfer of a Non-indigenous Species Via a Potentially Unsuitable Environment

The smooth shelled blue mussel, Mytilus galloprovincialis Lmk (Bivalvia: Mollusca) arrived in Pearl Harbor, Oahu, Hawai'i on 22 June 1998 as a member of the fouling community of the USS Missouri, and mussel spawning activity was observed within 2h of the vessel's arrival. Small mussels (<10mm shell length, approximately 6 weeks post-metamorphosis) were collected on 30 September 1998 from a submarine ballast tank in Pearl Harbor, indicating that a successful recruitment event had taken place very soon after the first arrival of the species at this location. We suggest that even if M. galloprovincialis is not able to establish permanently within Pearl Harbor, the fact that it has been able to successfully spawn and recruit to another shipping vector within the Harbor indicates that a stepping stone model of range expansion from temperate to temperate region via an intermediary subtropical environment is quite feasible for this species. Data from worldwide distributions of mussels of the family Mytilidae indicate that preferred habitats are eutrophic continental shelf regions, which suggests that successful establishment within Pearl Harbor is possible. However, oceanic coral-reef environments are not preferred habitat types, suggesting that M. galloprovincialis is not likely to become widely distributed in the Hawaiian Islands.