Population structure and reproduction of the alvinocaridid shrimp Rimicaris exoculata on the Mid-Atlantic Ridge: variations between habitats and vent fields

Rimicaris exoculata is a dominant species of deep Mid-Atlantic Ridge (MAR) vent fields and inhabits areas close to vent emissions at depths below 2000 m. Its high abundance and strong genetic connectivity along the MAR point at a remarkable ability to produce dispersing larval stages. However, the reproduction of this species long remained enigmatic because brooding females were rarely observed. Here, we describe the population structure and reproduction of R. exoculata at the Snake Pit and TAG vent fields (3600m depth) for the months of January-February. We observed major differences in population structure between habitats within a single vent field: females widely dominate the large swarms near active venting while inactive peripheries are inhabited by large males. Low temperature diffusion zones are mainly colonized by small juveniles of R. chacei instead of R. exoculata. Size structure of populations from dense active areas is polymodal at both fields, suggesting discontinuous recruitment. Male and female sizes did not vary across habitats and vent fields, with sexually mature female being slightly larger than males. In contrast to previous studies, hundreds of ovigerous females were observed at both vent fields, suggesting seasonal reproduction. Proportion of ovigerous females among sexually mature females were similar between vent fields (36.7 %). However, reproductive output was lower at TAG, where ovigerous females had smaller size-specific fecundity and egg size, and more aborted broods. Broods were colonized by the dirivultid copepod Stygiopontius pectinatus at both vent fields, apparently without deleterious effect on egg development. In the light of the observed variability in R. exoculata population structure, we propose a hypothetical scenario depicting its mating system and brooding behavior, and discuss more generally intraspecific interactions during its benthic life stages.