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 arc: Rimicaris loihi also occurs near Hawaii and R. cambonae is present on the Tonga Arc. These two species biogeographically co-occur and are genetically similar, raising questions about their speciation mechanism, how they maintain distinct species, and whether interbreeding occurs. Here, we use barcoding and shotgun sequencing to test their genetic isolation and investigate their speciation process. 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. We also observed regional-scale genetic structure among R. loihi populations from the Izu-Bonin-Mariana volcanic arc despite their broad geographic distribution. Finally, we found concomitant variations of shrimp densities and genetic diversity following fluctuations in geological and venting activities over a decade. A combination of geological instability, ocean currents dynamics and sea-level changes might drive temporary isolation among these local populations. We suggest similar factors, with longer isolation periods, may also have promoted speciation between the two Rimicaris species whereas distinct life-history traits could strengthen and maintain reproductive barriers. Overall, our results indicate different connectivity patterns on a volcanic arc that contrast with those observed previously at vents from mid-ocean ridges or back-arc basins systems.