GENETIC DIFFERENTIATION OF DEEP-SEA HYDROTHERMAL VENT ALVINELLID POPULATIONS (ANNELIDA, POLYCHAETA) ALONG THE EAST PACIFIC RISE

The alvinellid polychaetes, which live in the hottest part of the deep-sea hydrothermal environment, have a nested island-like distribution and locally are subjected to extinctions. They are sedentary and exhibit a peculiar reproductive behaviour and a development which may result in little or no planktonic stage (i.e. larval dispersal). The genetic variation within and among populations of the three main species (Alvinella pompejana, Alvinella caudata and Paralvinella grasslei) inhabiting vents along the East Pacific Rise was examined at a hierarchy of spatial scales using allozyme electrophoresis. The genetic diversity of P. grasslei is high (H-0 = 0.24), about twice that of both the Alvinella species (H-0 = 0.10). The three species show a strong tendency towards a heterozygote deficiency which systematically occurs at the same loci in nearly all the populations. These structures are particularly obvious in the genus Alvinella and might be explained by differential allozyme fitness. Populations display considerable genetic differentiation at the microgeographical scare, which could be explained by repeated founder effects in populations, but it varies from species to species according to their possible ability to be transported by crabs from vent to vent. However, the genetic variation among populations separated by at least 1000 km is of the same magnitude as that found within the 13 degrees N/EPR segment. These results demonstrate that each species maintains its genetic identity along the oceanic rifts despite the evidence for founder effects. To explain this phenomenon, we hypothesize that in such a harsh environment, genetic drift in alvinellid populations could be balanced by a uniform selective pressure stemming from the vent chemistry.