EXTENSIVE GENE FLOW AMONG MYTILID (BATHYMODIOLUS-THERMOPHILUS) POPULATIONS FROM HYDROTHERMAL VENTS OF THE EASTERN PACIFIC

Prior studies of the hydrothermal vent mussel Bathymodiolus thermophilus (Bivalvia: Mytilidae)), provided conflicting predictions about the dispersal ability and population structure of this highly specialized species. Analyses of morphological features associated with its larval shells revealed a feeding larval stage that might facilitate dispersal between ephemeral vent habitats. In contrast, an allozyme study revealed substantial genetic differentiation between samples taken fi-om populations 2370 km apart on Galapagos Rift (Latitude 0 degrees N) and the East Pacific Rise (13 degrees N). To resolve the discrepancy between these studies, we examined allozyme and mitochondrial (mt) DNA variation in new samples from the same localities plus more recently discovered sites (9 degrees and 11 degrees N) along the East Pacific Rise. Although analysis of 26 enzyme;determining loci revealed relatively low levels of genetic variation within the five populations, no evidence existed for significant barriers to dispersal among populations. We estimated an average effective rate of gene flow (Nm) of similar or equal to 8 migrants per population per generation. Two common mtDNA variants predominated at relatively even frequencies in each population, and similarly provided no evidence for barriers to gene flow or isolation-by-distance across this species' known range. Larvae of this species appear to be capable of dispersing hundreds of kilometers along a continuous ridge system and across spreading centers.