Cryptic species of Clavelina (Ascidiacea) in two different habitats: harbours and rocky littoral zones in the northwestern Mediterranean

Marinas and harbours provide ideal sites for the study of population geneti cs of marine invertebrates with restricted dispersal capabilities. They com bine a confinement effect, particular ecological conditions (pollution, tur bidity), and the possibility of high gene flow through ship-borne propagule s, which greatly increases the natural dispersal capability of sexual and a sexual propagules in many species with short-lived larvae. We studied the g enetic structure of populations of the ascidian Clavelina lepadiformis livi ng inside and outside harbours in the north-western Mediterranean. A 500-bp segment of the cytochrome c oxidase subunit I (COI) mitochondrial gene was sequenced in three populations from inside harbours (interior form) and in three populations from the rocky littoral (exterior form). Two congeneric Mediterranean species, Clavelina sp. and C. dellavallei, were used for comp arison. We found that the interior and exterior forms of C. lepadiformis be long to two distinct clades, with a genetic divergence of 5%. Gene-flow val ues among these forms were insignificant. The lack of gene flow and the gen etic divergence suggest that the interior and exterior forms of C. lepadifo rmis are in fact cryptic species rather than differentiated populations of the same species. Levels of gene flow were higher among interior habitats t han among exterior habitats, a pattern likely maintained by genetic exchang e through ships. We discuss the possible origins of the present-day distrib ution of these cryptic species. We contend that the study of species living both inside and outside these particular habitats will reveal more instanc es of genetic discontinuities allowing local adaptations.