Genetic diversity of the Ruditapes decussatus and evidence of its hybridization with the alien R. philippinarum in the Western Mediterranean Sea

The introduction of alien species in marine ecosystems is often driven by the increasing demand of fishery resources. This is the case of the Manila clam (R. philippinarum), imported in Europe from Japan since the 1970s, to meet the growing demand for clams that the native species, the grooved carpet shell clam (Ruditapes decussatus), could not satisfy. Alien species introduction could threaten the genetic diversity and integrity of the native clam, also causing hybridization (i.e., gene flow from one species into the gene pool of another). Since R. philippinarum recently spread in a few important Mediterranean coastal areas, a combined approach based on morphological characteristics, length differences of two nuclear species-specific markers (ITS2, 5SrDNA) and the sequence of the mitochondrial gene cytochrome c oxidase subunit I (COI), was used to investigate the presence of hybrids in six Mediterranean wetlands (Sardinia, Italy). Eight individuals morphologically identified as R. decussatus were hybrids, having sequences specific to both R. decussatus and R. philippinarum in their nuclear DNA (ITS2 and 5SrDNA). Most of these individuals were found to be post-first generation (F1) hybrids indicating that F1-hybrids may be fertile. Secondly, to study the genetic diversity of R. decussatus in the Sardinian wetlands as well as in its whole distribution area, >380 new COI sequences from the eastern Atlantic Ocean and Mediterranean Sea were analysed along with those available from public databases. Mitochondrial COI data revealed variable haplotype and nucleotide diversities in different areas, which were not dependent on sample sizes. The aquaculture breeding activities and clam transplantation between different countries, along with the long pelagic larval dispersal and the commercial import of other bivalve species might have promoted gene exchange between different sites and thus higher diversity levels in a few wild populations. Our research, evaluating the genetic makeup of wild and hatchery stocks and clarifying the degree of hybridization, can contribute to develop further recommendations for conserving the genetic integrity of R. decussatus.