A tale of two shrimps – Speciation and demography of two sympatric shrimp species from hydrothermal vents

Hydrothermal vents can serve as 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 biogeographically co-occur and are genetically highly similar, raising the question of the mechanisms of how they maintain distinct species, including the possibility of interbreeding. Here, we use barcoding and shotgun sequencing to test if hybridization is occurring. We also evaluate population demography over 10 years to assess population densities and sex ratios at 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 sex ratios between species populations suggest that life-history traits such as different mating systems might reinforce reproductive barriers. We observed fluctuations in shrimp densities alongside their genetic diversity, linked to variations in hydrothermal activity over years. We suggest that geographic isolation after a rare, historical long-distance dispersal across volcanic arcs, followed by environmentally-driven genetic fluctuations may have fostered rapid divergence by allopatry, although sympatric speciation is not excluded.