Population genetics, gene flow, and biogeographical boundaries of Carcinus aestuarii (Crustacea: Brachyura: Carcinidae) along the European Mediterranean coast

Carcinus aestuarii Nardo, 1847 is a widespread coastal crab species throughout the Mediterranean Sea with a pelagic larval phase. This species tolerates a wide range of environmental conditions and typically inhabits fragmented habitats, such as embayments, lagoons and estuaries. It is therefore a good candidate species for studying and testing different phylogeographical hypotheses in the Mediterranean Sea. By contrast to its Atlantic sister species, Carcinus maenas, studies on the population genetic structure of C. aestuarii in its native range are still scarce. In the present study, specimens from along the European Mediterranean Sea were collected and DNA-sequenced and analyses were applied to discriminate between present day and historical factors influencing the population genetic structure of this species. The results obtained demonstrate the existence of two genetically distinct geographical groups, corresponding to the eastern and western Mediterranean, with further subdivision within the East Mediterranean Basin. A strong asymmetric gene flow was recorded toward the Eastern Basin, which may play a crucial role in shaping the present day biogeographical patterns of this species and potentially other sympatric ones with pelagic larvae.We are indebted to Sara Fratini, Carsten Müller, Yvan Perez, Pere Abelló, Florian Gmeiner, Silke Reuschel, Ruth Jesse, Sebastian Klaus, and Henrik M. Schubart, as well as several students of the University of Regensburg for their help in collecting and sequencing specimens of C. aestuarii. We thank three anonymous reviewers for their helpful comments. Travel for this study was partly financed through DAAD exchange programmes to Italy (VIGONI D/04/ 47157 thanks to Marco Vannini) and Spain (Acciones Integradas Hispano-Alemanas D/03/40344 thanks to JoséA. Cuesta). The authors declare that there are no conflicts of interest.publishe