Population structure and recruitment in mytilid bivalves from the Lucky Strike and Menez Gwen hydrothermal vent fields (37°17' N and 37°50' N on the Mid-Atlantic Ridge)

Abstract

Population structure and recruitment pattern of undescribed mytilid bivalves that dominate the animal communities associated with vents of the Lucky Strike and Menez Gwen hydrothermal vent fields (37°17' N and 37°50' N on the Mid-Atlantic Ridge) were studied from size-frequency analyses. Intra-site comparisons reveal that length-frequency distributions differ significantly from one location to the other. This spatial segregation of the sizes could be due to spatio-temporal variability in environmental conditions and to intraspecific competition processes, especially adult-larval interactions (competition for space and food resources, intraspecific larviphagy) that could prevent larvae from settling on surfaces covered by adults. A modal decomposition analysis was conducted after pooling samples collected from different locations at the same site. Polymodal structure of length-frequency distributions indicates that recruitment is likely discontinuous in time. The presence of postlarvae in each population of the Lucky Strike vent field suggests that a recruitment event took place just before 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 displays similar modal values, especially for the smallest mode. This suggests a synchronization of the recruitment between the 2 vent fields, which are separated by a distance of 60 km. No seasonal pattern in recruitment could be identified although newly recruited individuals were observed in June 1993 on Lucky Strike sites.