Gene flow among vestimentiferan tube worm (Riftia pachyptila) populations from hydrothermal vents of the eastern Pacific

Allozyme data are presented for six discrete populations of the giant hydrothermal vent tube worm Riftia pachyptila Jones, 1981 collected throughout the species' known range along mid-ocean spreading ridges of the eastern Pacific Ocean. Contrary to an earlier report, levels of genetic variation are relatively high in this species. Estimates of gene flow based on F-statistics revealed that dispersal throughout the surveyed region is sufficiently high to counter random processes that would lead to losses of genetic diversity and significant population differentiation. R. pachyptila, like other species of tube worms, displays considerable morphologic variation among populations, but this diversity is not reflected in allozyme variation. Vestimentifera, in general, appear to show extensive phenotypic plasticity. In the light of the available genetic data, caution is warranted when making inferences about the taxonomic status of collections based on morphological variation alone. A general decrease in estimated rates of gene flow between geographically more distant populations supports the hypothesis that dispersal in this species follows a stepping-stone model, with exchange between neighboring populations in great excess of long-distance dispersal. High levels of gene flow have been recorded in a variety of vent fauna and may be a prerequisite for success of species found in the ephemeral habitats associated with regions of sea-floor hydrothermal activity.