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 genetics of marine invertebrates with restricted dispersal capabilities. They combine a confinement effect, particular ecological conditions (pollution, turbidity), and the possibility of high gene flow through ship-borne propagules, which greatly increases the natural dispersal capability of sexual and asexual propagules in many species with short-lived larvae. We studied the genetic structure of populations of the ascidian Clavelina lepadiformis living 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 comparison. We found that the interior and exterior forms of C. lepadiformis belong to two distinct clades, with a genetic divergence of 5%. Gene-flow values among these forms were insignificant. The lack of gene flow and the genetic divergence suggest that the interior and exterior forms of C. lepadiformis are in fact cryptic species rather than differentiated populations of the same species. Levels of gene flow were higher among interior habitats than among exterior habitats, a pattern likely maintained by genetic exchange through ships. We discuss the possible origins of the present-day distribution of these cryptic species. We contend that the study of species living both inside and outside these particular habitats will reveal more instances of genetic discontinuities allowing local adaptations.
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