The soft-shell clam Mya arenaria is probably the first marine invasive organism in Europe, presumably introduced from the Atlantic coasts of North America by the Vikings in the 13th century (Petersen et al. 1992; Strasser 1999). Nowadays it inhabits commonly shallow-water sandy and muddy-sandy bottom habitats of the Northern Hemisphere. The species possesses several biological and ecological attributes that enable it to colonize new habitats, including high fecundity, pelagic larval development (pelagic larval duration estimated to be 10-35 days), rapid growth, and ability to withstand a very wide range of environmental conditions. An additional key feature of this species is its long life span (with individuals older than 27 years). Despite these features, M. arenaria is unexpectedly characterized by a relatively low level of genetic polymorphism, as compared to other marine bivalves, both in its native and introduced range (Lasota et al. 2004; Strasser et al. 2009). One possible explanation of this low genetic polymorphism is a particular demographic process: populations of M. arenaria, both in native and colonized habitats, often show specific age and size structures with only adult specimens, suggesting long episodes without recruitment of new individuals. Recruits might thus arrive only after drastic environmental changes leading to demographic processes involving recurrent population bottlenecks which could explain the limited genetic polymorphism of this species. At the same time, spatial genetic structure is also very low in the soft-shell clam at a regional scale, which may reflect high dispersal potential (pelagic larvae). This observation is however against the hypothesis of bottleneck at a local scale (bottlenecks being efficient drivers of genetic structure between populations). The spatial scale at which dispersal and recruitment operate (a large one at metapopulation scale) may explain this apparent discrepancy.

 

To better understand the factors which control the genetic polymorphism in Mya arenaria,  detailed investigations of population dynamics processes are needed. The overall objective of this project is to study demography and connectivity in Mya arenaria populations by combining ecological, molecular and biogeochemical approaches.

 

To fulfill these objectives, three main topics will be addressed:

(1) population dynamics of a natural population, based on age/size structure analyses, recruitment monitoring and study of larval supply

(2) genetic diversity of the larval pool over the reproductive season, as compared to adults

(3) study of the connectivity by means of different tools at the scale of the Gulf of Gdansk (Southern Baltic Sea)

 

(1) This first task will be conducted by monitoring one natural population of M. arenaria of Puck Bay, a semi-enclosed basin in the Southern Baltic Sea. Both the benthic and pelagic phases will be studied. The benthic phase will be studied based on age/size structure analyses and recruitment monitoring. Beside field analyses, which will involve work onboard research vessel, laboratory experiments on settlement will be performed. Larval abundances will be monitored in parallel to the recruitment dynamics, to determine if recruitment is controlled by larval supply. The studied area in the Baltic sea will facilitate this work as only four bivalve species are recorded, thus limiting problems to identify larvae of M. arenaria (e.g. Strasser and Günther 2001). If necessary, in some cases, the molecular markers used in part (2) would allow confirming morphological diagnostic for larval identification (e.g. Morgan and Rogers 2001).

 

(2) Marine invertebrates, in general, have extremely large variance in individual reproductive success and high larval mortality, resulting in a number of individuals participating to the next generation (effective number of breeders) much lower than the actual number of reproductive adults. This so-called "Sweepstakes Reproductive Success" (SRS) hypothesis leads to increased genetic drift among generations, and may also explain star-shaped phylogenies of mtDNA sequences (Hedgecock and Pudovkin 2011), an additional characteristic of M. arenaria (Strasser et al. 2009).

The objective is to study the genetic diversity of the larval pool of M. arenaria, as compared to that of the adults and recruits, to test if different larval cohorts show different genetic composition (i.e. are produced by a different set of breeders) and if larvae and juveniles have a reduced genetic diversity as compared to adults, two predictions of the SRS hypothesis. In addition, molecular tools will allow to check for the existence of kin-related larvae, another expectation under SRS. Such data are quite rare in broadcast spawners.

The molecular markers necessary to conduct this study already exist: 12 microsatellite loci have been recently developed by the applicants (Lasota, Daguin-Thiébaut & Viard, unpublished results).

 

(3) In marine invertebrate species, long-lived larvae ensure population connectivity that control gene flow (Cowen et al. 2007). To study connectivity in M. arenaria, molecular markers (as commonly used in other species, e.g. Hedgecock et al. 2007) may be difficult to use due to the lack of genetic structure between populations. As an alternative, connectivity at the scale of the Gulf of Gdansk (inner and outer Puck Bay and Vistula mouth) and neighbouring populations will be studied using biogeochemical markers that have been found reliable tags in M. arenaria (Strasser et al. 2008a; 2008b). To determine the natal origin of early settlers of M. arenaria, larvae will be produced in the laboratory and cultured at sites harboring adult populations, both within and outside the Gulf of Gdansk, following Becker et al. (2007). This will allow to determine the chemical signature of larval shells (prodissoconchs). The chemical composition of the retained prodissoconch from juveniles of the Gulf of Gdansk will be compared to these elemental fingerprints to determine their origin.

 

Consortium description:

This PhD project is part of a broader project which aims at studying molecular evolution and ecology of the soft-shell clam, which involves both applicant teams which already collaborate for 2 years, thanks to an EGIDE-funded project for supporting travels between the two labs and a research project funded by the Polish Ministry for education and scientific research, providing complementary approaches in solving above scientific problems.

The Institute of Oceanography of the University of Gdansk is a leading oceanographic institution in Poland. We will provide excellently equipped laboratory facilities, including the field station and the research vessel. The team “Laboratory of Estuarine Ecology” belonging to the“Department of Marine Ecosystems Functioning” has already years of experience in ecology and population genetics of marine bivalves, including M. arenaria.

The team "Diversity and connectivity in coastal marine landscapes" (DIVCO) is part of the department "Adaptation and diversity in marine systems" of the CNRS and University Pierre et Marie Curie (UPMC, Paris). The core research theme of DIVCO is the study of dispersal processes in coastal marine organisms and their consequences on the diversity of populations and communities, focusing on introduced/invasive species. The team has years of experience in evolutionary ecology and ecology of marine mollusc larvae (e.g. Crepidula fornicata).

The PhD work will be supervised by two young researchers (Rafal Lasota and Thierry Comtet) with the co-supervision of Maciej Wolowicz and Frédérique Viard, two senior scientists (both habilited to supervise PhD students). Each of these four researchers have different areas of expertise that will benefit to the PhD work. The two labs will also provide complementary equipment and facilities to the PhD students to achieve the proposed objectives of this work.

 

Selected references concerning the topic: