Genetic diversity of oceanic island Lasaea (Mollusca: Bivalvia) lineages exceeds that of continental populations in the northwestern Atlantic

Direct-developing lineages of the intertidal marine bivalve Lasaea have colonized both upstream mainland (southeastern Florida) and downstream oceanic island (Bermuda) locations in the western North Atlantic. Replicate samples from these two regional populations, separated by about 1500 km of open ocean, were sequenced for a 655-nucleotide portion of the mitochondrial (mt) cytochrome oxidase subunit I gene. Twelve haplotypes (2 Floridian and 10 Bermudan) were obtained that differed by a maximum of four substitutions among pairwise comparisons. Phylogenetic analysis yielded a parsimony network within which the mainland lineages clustered in one of the terminal branches; a mirror image of a priori expectations based on regional surface-current polarity. It is difficult, however, to envisage a plausible countercurrent dispersal mechanism. This tree topology may stem from divergent demographic processes operating on these two evolutionarily recent regional populations. The starlike phylogenetic pattern of Bermudan lineages is consistent with a history of rapid population growth. The restricted genetic repertoire and relative ecological scarcity of Floridian lineages imply either a recent founder event by unstudied Caribbean source populations or else a history of pronounced bottlenecks in population size. Bermuda's impoverished Caribbean marine biota may allow western North Atlantic Lasaea lineages to escape severe competitive interactions impacting other parts of their geographic range.