The distribution and molecular diversity of the Eastern Atlantic and Mediterranean chthamalids (Crustacea, Cirripedia)

The three chthamalids Chthamalus stellatus, C. montagui and Euraphia depressa are common inhabitants of the intertidal zone in the Eastern Atlantic, Mediterranean Sea and Black Sea. In this study, we investigated the occurrence of these barnacles in a wide range of their distribution. Population divergences of these two species have been inferred using three molecular markers - internal transcribed spacer (ITS), elongation factor 1 alpha (EF-1 alpha) and cytochrome oxidase subunit I (COI). ITS sequences of C. stellatus were identical throughout the species range, whereas ITS sequences of C. montagui indicated that the Black Sea and Mediterranean populations are isolated from the Atlantic population. The COI and EF-1 alpha sequences were the most variable and informative. They indicated a high genetic divergence between Atlantic, Mediterranean and Black Sea populations for C. montagui. In addition significant genetic structure was found among the populations of C. stellatus based on EF-1 alpha but not COI. Interestingly, our molecular dating analysis correlated the pattern of diversification in C. montagui to major geological changes that occurred in the Mediterranean during the end of the Messinian and Pleiocene periods. We suggest that palaeohistory shaped the divergences between Chthamalus populations that have probably been maintained by current hydrographic conditions. Finally, COI phylogenetic analysis placed the genus Euraphia within the Chthamalus clade, suggesting the need for a taxonomic revision of Euraphia. This study represents the most detailed phylogeographical analysis of intertidal Mediterranean species to date, and shows that geological events have strongly shaped the current diversity pattern of this fauna.