GENETIC DIFFERENTIATION ACROSS THE WESTERN PACIFIC POPULATIONS OF THEHYDROTHERMAL VENT BIVALVE BATHYMODIOLUS SPP AND THE EASTERN PACIFIC (13-DEGREES-N) POPULATION OF BATHYMODIOLUS-THERMOPHILUS

Deep-sea mussels, closely related to the Bathymodiolus genus and indiv iduals of Bathymodiolus thermophilus, were collected on hydrothermal v ents of the Lau and North-Fiji back-are basins (Western Pacific) and a t 13 degrees N on the East Pacific Rise, respectively. Separate tissue homogenates were submitted to starch gel electrophoresis in order to study the genetic structure of the populations at 11 enzyme loci. Sign ificant departures from the Hardy-Weinberg equilibrium were found at A at-1, Gpi and Pgm loci without any location specificity. All the obser ved departures corresponded to heterozygote deficiencies that are comm on in mollusc bivalves. The genetic variability of these mussels was l ow in all samples, especially for the North-Fiji population. The genet ic distances measured between populations demonstrated that significan t genetic differentiation occurs between the Western Pacific mussel po pulations and the Eastern Pacific B. thermophilus, whereas gene flow a ppeared to be maintained between the two back-are basins. The presence of both shared heterozygotes and genetic distances found between the western and the eastern populations do not allow us to separate these allopatric forms as distinct sibling species.