| Summary: | In order to elucidate the evolutionary history and the population structure of the membersof the phylum Cycliophora, which live commensally on three species of lobsters, we studiedsequence variation in the mitochondrial gene cyctochrome c oxidase subunit I. Overall 242sequences from 16 locations on both coasts of the North Atlantic, including the North Seaand the Mediterranean, were analysed, revealing 28 haplotypes, with a maximum sequencedivergence of 16.6%. Total genetic diversity was high ( h = 0.8322, π π = 0.0898), as it was forthe commensals on Homarus americanus (17 haplotypes, h = 0.7506, π π = 0.0504). However, itwas low for commensals on Nephrops norvegicus (6 haplotypes, h = 0.3899, π π = 0.0035), andintermediate for cycliophorans on Homarus gammarus (5 haplotypes, h = 0.3020, π π = 0.0140).Although two of the host lobsters co-inhabit the coastal waters of Europe, a strong geneticstructure (78.45% of the observed genetic variation) was detected among populations on allhost species, indicating the existence of a reproductively isolated species on each lobster.In addition, genetic structure over long distances exists among populations on each hostspecies. Such patterns can be explained by the limited dispersal ability of the cycliophoranchordoid larva. Demographic and phylogenetic analyses suggest old and possibly crypticpopulations present on H. americanus and H. gammarus , while the latter may have experiencedrecent bottlenecks, perhaps during Pleistocene glaciations. Populations on N. norvegicus appear to be of recent origin.
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