GENETIC DIFFERENTIATION ACROSS THE WESTERN PACIFIC POPULATIONS OF THE HYDROTHERMAL 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 individuals of Bathymodiolus thermophilus, were collected on hydrothermal vents of the Lau and North-Fiji back-are basins (Western Pacific) and at 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. Significant departures from the Hardy-Weinberg equilibrium were found at Aat-1, Gpi and Pgm loci without any location specificity. All the observed departures corresponded to heterozygote deficiencies that are common in mollusc bivalves. The genetic variability of these mussels was low in all samples, especially for the North-Fiji population. The genetic distances measured between populations demonstrated that significant genetic differentiation occurs between the Western Pacific mussel populations and the Eastern Pacific B. thermophilus, whereas gene flow appeared 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.