Population structure and environmental niches of Rimicaris shrimps from the Mid Atlantic Ridge

Among the endemic and specialized fauna from hydrothermal vents, Rimicaris shrimp surely constitutes one of the most important and emblematic components of these ecosystems. In the Mid Atlantic Ridge, two species affiliated to this genus co-occur: Rimicaris exoculata and Rimicaris chacei that differ by their morphology, their trophic regime as well as by their abundance. The first forms large and dense aggregations on active vent chimney walls in close proximity to vent fluid emissions, whereas the second is recognized as much less conspicuous, living mostly in scattered groups or solitary further away from the fluids. However, the recent revision of Rimicaris juvenile stages from the Mid Atlantic Ridge shows that R. chacei abundance would be higher than expected at these early life stages. Here, we describe and compare the population structures of R. exoculata and R. chacei at the Snake Pit and TAG vent fields. We show distinct population demographics between the two co-occurring shrimps with a large post settlement collapse in R. chacei population suggesting a large juvenile mortality for this species. We also observe important spatial segregation patterns between the two species and their different life stages. Additionally, our results highlight distinct niches for the earliest juvenile stages of both R. exoculata and R. chacei, compared to all the other life stages. Finally, we discuss the potential factors - predation and competitive interactions among others - that could explain the differences we observe in the population structure of these two species.