Recent population expansion and connectivity in the hydrothermal shrimp Rimicaris exoculata along the Mid-Atlantic Ridge

Aim: Deep-sea hydrothermal vents are unstable habitats that are both spatially and temporally fragmented. In vent species, a ‘short-term insurance’ hypothesis would lead us to expect mostly self-recruitment, limiting the loss of larvae in the deep ocean or water column and increasing genetic differentiation over the time elapsed since colonization. Alternatively, a ‘long-term insurance’ hypothesis would support the prediction of selection for large-scale dispersal, to ensure longterm persistence in these ephemeral habitats. The main goal of this study was to infer the spatial and temporal distribution of genetic diversity of the shrimp Rimicaris exoculata, which forms high-density local populations on hydrothermal vents along the Mid-Atlantic ridge. Location: Deep-sea hydrothermal vents along the Mid-Atlantic Ridge. Methods: We used sequences of mitochondrial cytochrome c oxidase subunit I (COI, 710 bp) to assess the spatio-temporal distribution of genetic diversity across five hydrothermal fields from 36 N to 4 S. Results: In contrast to previous results from pioneer studies, very high haplotype diversity was observed in vents across the entire region (i.e. 0.69–0.82), indicating current large effective population size and low drift. The star-like shape of the network of haplotypes, the lack of spatial genetic structure and the significance of tests reflecting demographic effects, together with the fitting of a population expansion model, all support a recent population expansion. Main conclusions: Our results suggest a very recent common history of R. exoculata populations/demes along the Mid-Atlantic Ridge, derived after a common bottleneck or founder event and followed by a concomitant demographic expansion. This study therefore suggests a large effective population size and/or high dispersal capacity, as well as a possible recent (re)colonization of Mid-Atlantic hydrothermal vents by R. exoculata