Complex genetic population structure of the bivalve Cerastoderma glaucum in a highly fragmented lagoon habitat

The genetic structure of a lagoon specialist, the bivalve Cerastoderma glaucum, was studied from 4 nuclear microsatellite loci and sequences from a 514 bp mitochondrial DNA region. Both marker types revealed strong differentiation among populations that is likely due to the highly fragmented distribution of this species. The geographic location and strength of the major divisions were different between the genomes. The deepest phylogeographic split in mtDNA grouped Aegean Sea and Ponto-Caspian region populations against the more western populations, whereas nuclear data singled out Ionian Sea populations or indicated a Ponto-Caspian character of nuclear DNA from the Ionian Sea. In agreement with their presumably rapid post-glacial establishment, the northern populations showed evidence of founder effects and strong genetic drift in their genetic structuring. Two unexpected geographic disjunctions were detected in the mtDNA, but not in the microsatellite characteristics, of the studied populations. We suspect long-distance dispersal via migrating birds to be a probable dispersal means among isolated lagoon habitats and to have generated the disjunctions by contributing to the post-glacial spread of C. glaucum to its northern distribution area. The discrepancies between the nuclear and mitochondrial genetic characteristics of the populations could be a result of selective sweeps in mtDNA, sex-biased dispersal, biased effective sex ratio or differential introgression of the genomes. The high level of differentiation found between the main genetic groups led us to conclude that the taxonomy of Cerastoderma needs revision, but we acknowledge that due to hybridization the taxa can be occasionally impossible to delineate.