Methou, P.; Johnson, S. B.; Sherrin, J.; Chen, C.; Tunnicliffe, V.

Hydrothermal vents are ideal natural laboratories to study speciation processes due to their fragmented distribution often with geographic barriers between habitats. Two sympatric species of Rimicaris shrimps occur at vents on the Izu-Bonin-Mariana volcanic arcs: Rimicaris loihi also found near Hawai\'i and R. cambonae also present on the Tonga Arc. These two species are genetically very close and sometimes co-occur, raising the question of how they can remain separate if interbreeding can still happen. Here, we use barcoding and shotgun sequencing to test if hybridization still occurs. We also evaluated population demography over 10 years to assess population densities and sex ratios at some vents. Our results support R. cambonae and R. loihi as two distinct species despite sympatry throughout part of their range and presence of hybrids. Different population sex ratios between species suggest that life-history traits such as different mating systems might reinforce reproductive barriers. We also observed fluctuations in shrimp densities alongside their genetic diversity, linked to variations in hydrothermal activities over the years. We suggest that geographic isolation after a rare, historical long-distance dispersal across volcanic arcs followed by such environmentally-driven genetic fluctuations may have fostered rapid divergence by allopatry, although sympatric speciation is not excluded.