POPULATION-STRUCTURE AND RECRUITMENT IN MYTILID BIVALVES FROM THE LUCKY STRIKE AND MENEZ GWEN HYDROTHERMAL VENT FIELDS (37-DEGREES-17'-N AND 37-DEGREES-50'-N ON THE MID-ATLANTIC RIDGE)

Population structure and recruitment pattern of undescribed mytilid bi valves that dominate the animal communities associated with vents of t he Lucky Strike and Menez Gwen hydrothermal vent fields (37 degrees 17 ' N and 37 degrees 50' N on the Mid-Atlantic Ridge) were studied from size-frequency analyses. Intra-site comparisons reveal that length-fre quency distributions differ significantly from one location to the oth er. This spatial segregation of the sizes could be due to spatio-tempo ral variability in environmental conditions and to intraspecific compe tition processes, especially adult-larval interactions (competition fo r space and food resources, intraspecific larviphagy) that could preve nt larvae from settling on surfaces covered by adults. A modal decompo sition analysis was conducted after pooling samples collected from dif ferent locations at the same site. Polymodal structure of length-frequ ency distributions indicates that recruitment is likely discontinuous in time. The presence of postlarvae in each population of the Lucky St rike vent field suggests that a recruitment event took place just befo re sampling in June 1994, and that mytilid recruitment is synchronized between sites, which is confirmed by the comparison of modal values. Length-frequency distribution of the Menez Gwen mytilid population dis plays similar modal values, especially for the smallest mode. This sug gests a synchronization of the recruitment between the 2 vent fields, which are separated by a distance of 60 km, No seasonal pattern in rec ruitment could be identified although newly recruited individuals were observed in June 1993 on Lucky Strike sites.