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

The shrimp Rimicaris exoculata is the most conspicuous component of vent communities developing around hydrothermal fluid emissions below 2000 m on the northern Mid-Atlantic Ridge (nMAR). Its high genetic connectivity suggests a remarkable ability to produce dispersing larval stages. However, so far brooding females have been rarely observed and reproduction remained enigmatic. Spatially complex population structures related to the heterogeneity of local habitat conditions are described for many vent species, this information being fundamental to gain a better understanding of their life history. Here our aim was to assess such complexity along with reproductive development in R. exoculata populations within two vent fields, TAG and Snake Pit (3620m and 3470m depth respectively). We compared samples collected in January–February 2014 among visually distinct assemblages with different degrees of exposure to vent fluids. Dense aggregations located near active venting included mostly females and immature individuals, while inactive peripheries harbored low density assemblages of large males. Small juveniles gathering around low temperature diffusions belonged to another species, Rimicaris chacei. One third of the sexually mature females were ovigerous at the two vent fields during our sampling period, with lower fecundities and egg sizes in the TAG population. Overall, the observed shrimp distribution patterns were consistent across both vent fields, although a high degree of heterogeneity in population structure was observed locally within dense aggregations, probably reflecting micro-scale variations in environmental conditions. Our results thus highlight spatially complex population structures where R. exoculata females brood eggs within dense aggregations exposed to vent fluids, while peripheral inactive areas may be important mating grounds for adults. In addition, we suggest temporal variability in reproductive activity, increasing in the winter season, which questions potential seasonality in a deep-sea species. • Shrimps show contrasting population structures between habitats within vent fields. • Females and immature individuals dominate dense R. exoculata aggregations. • Scattered shrimps in the inactive vent peripheries are mostly R. exoculata males. • One third of the sexually mature females were brooding in winter 2014. • R. exoculata broods near vent fluids, whereas mating may occur in the periphery. [ABSTRACT FROM AUTHOR]