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 exhib it a peculiar reproductive behaviour and a development which may resul t in little or no planktonic stage (i.e. larval dispersal). The geneti c variation within and among populations of the three main species (Al vinella pompejana, Alvinella caudata and Paralvinella grasslei) inhabi ting vents along the East Pacific Rise was examined at a hierarchy of spatial scales using allozyme electrophoresis. The genetic diversity o f P. grasslei is high (H-0 = 0.24), about twice that of both the Alvin ella species (H-0 = 0.10). The three species show a strong tendency to wards a heterozygote deficiency which systematically occurs at the sam e loci in nearly all the populations. These structures are particularl y obvious in the genus Alvinella and might be explained by differentia l allozyme fitness. Populations display considerable genetic different iation at the microgeographical scare, which could be explained by rep eated founder effects in populations, but it varies from species to sp ecies according to their possible ability to be transported by crabs f rom vent to vent. However, the genetic variation among populations sep arated by at least 1000 km is of the same magnitude as that found with in the 13 degrees N/EPR segment. These results demonstrate that each s pecies maintains its genetic identity along the oceanic rifts despite the evidence for founder effects. To explain this phenomenon, we hypot hesize that in such a harsh environment, genetic drift in alvinellid p opulations could be balanced by a uniform selective pressure stemming from the vent chemistry.