High gene flow in Atlantic vent shrimp species with multiple ridge colonizations by ecotypic lineages with contrasting symbioses

Conservation concerns regarding potential mineral extraction near deep-sea hydrothermal vents have emphasised the need for a better understanding of the contemporary connectivity of various vent-endemic species, accounting for the specifics of their life-history traits and habitat requirements. Rimicaris shrimps are a major component of vent communities in the Atlantic Ocean, both along the Mid-Atlantic Ridge (MAR) and at the Mid-Cayman Spreading Center (MCSC). Using ddRAD sequencing, we examined the genetic differentiation of three Atlantic Rimicaris species to better understand their past and present migration pathways and the possible demographic history that led to their current distribution and ecology. Rimicaris exoculata and R. chacei inhabit the MAR and exhibit widespread gene flow along the ridge with no areas of restricted connectivity. R. hybisae occurs at the MCSC and exhibits almost no genetic difference with its congeneric from the MAR, R. chacei, despite the geographic distance between them and their contrasting ecologies and morphologies. Demographic inferences support a scenario of a very recent (less than 2000 generations) separation between MCSC and MAR shrimp populations, followed by a rapid expansion in both regions, possibly triggered by symbiotic development. Analyses further suggest recent or ongoing secondary contacts involving two-way migration, which is more pronounced from the MCSC towards the MAR. All together these results suggest that R. chacei and R. hybisae represent distinct forms of the same species, which we hereby synonymise as R. chacei, highlighting their morphological plasticity in relation to symbiosis.