Impacts of supplementation aquaculture on the genetic diversity of wild Ruditapes decussatus from northern Spain

The European carpet shell Ruditapes decussatus is among the most important shellfish species cultured in estuaries and bays of the northern Iberian Peninsula. Increased demand and overfishing of native R. decussatus, as well as irregular yields due in part to pollution and urban sprawl, have led to overexploitation of the species and to the import of the non-indigenous R. philippinarum in the early 1980s. In recent years, supplementation aquaculture of R. decussatus (supportive breeding) has been implemented as a management tool to recover the exhausted native carpet shell populations in Asturias, northern Spain. In this study, taxonomic analysis and a suite of genetic markers (ITS2, 5SrDNA, and mtCOI) were employed to assess the genetic composition of R. decussatus in 2 estuaries of the region, Ra del Eo and Ra de Villaviciosa. Our results revealed species misidentification and evidenced genetic introgression from the non-indigenous R. philippinarum into wild R. decussatus samples (4.2%), as well as in the aquaculture seed used for population supplementation (2.0%). Furthermore, we found significant genetic differentiation (F ST = 0.03750, p < 0.0001) between wild populations and aquaculture seed, while the adults of both estuaries were genetically homogeneous (p > 0.05). Several actions are strongly recommended to prevent inadvertent introduction of non-indigenous genomes and other genetic erosive processes on native species in the region: (1) banning releases of the alien species R. philippinarum, (2) developing diagnostic tests for detection of hybrids in captive breeding programs, and (3) assessing the conservation status of wild R. decussatus populations.