Phylogenetic relationships of mid-oceanic ridge and continental lineages of Lasaea spp. (Mollusca : Bivalvia) in the northeastern Atlantic

Direct-developing lineages of the genus Lasaea are common constituents of b oth oceanic island and continental rocky shore crevice faunas in the easter n North Atlantic. We utilized mitochondrial gene sequence variation to fles h out the phylogenetic relationships of individuals sampled from 2 Macarone sian archipelagos (Azores, Madeira) and from downstream continental (Iberia n) sites. There was no evidence for colonization of the islands by upstream western North Atlantic congeners. Of 5 Lasaea clades detected in Iberia, 1 was also present on Madeira, whereas 4 of the 5 had representatives on the Azores. Madeira did not share haplotypes with the other sampling locations . In contrast, the Azorean and Iberian samples shared multiple haplotypes a nd our phylogenetic tree topologies were consistent with a minimum of 6 inf erred migration events across the > 1400 km oceanic expanse separating thes e 2 regional populations. Three of the putative migration events involved a predominantly island clade whose topology was consistent with colonization by ancestral continental lineages, extensive island cladogenesis, and seco ndary downstream migrations back to the mainland. The remaining 3 inferred migration events were distributed across the tips of the phylogenetic trees , a topology consistent with evolutionarily recent migrations against the p revailing current fields. Our results indicate that the pattern of easterly surface flow in the study area may generate differentially effective dispe rsal filters downstream of the Azorean and Madeiran arckipelagos. Evidence for countercurrent migration in marine populations should be assessed in li ght of the totality of surface-flow patterns in the study system, not merel y the prevailing one.