Endogenous versus exogenous factors: What matters for vent mussel communities?

International audienceThe factors influencing the structure of hydrothermal communities are not necessarily similar across geographical regions and vent fields. In the present study, we explore the role of environmental conditions on vent community structure at local and regional spatial scales, focusing on the assemblages dominated by the engineer species Bathymodiolus azoricus. Their presence in several vent fields on the northern Mid-Atlantic Ridge (MAR) represents a unique opportunity to compare their associated fauna and to better understand large-scale biodiversity patterns. Two types of factors were considered: exogenous ones such as the presence of biogeographical barriers, depth, and distance between sites, and those linked to the intrinsic characteristics of each field (endogenous factors, e.g. local abiotic conditions). The main goals of our study are to describe and compare the biological and environmental characteristics of mussel assemblages collected from three vent fields - Menez Gwen (MG), Lucky Strike (LS) and Rainbow (RB) - located in the Azores Triple Junction on the MAR. These fields differ in the chemistry of their end-member fluids, their depths as well as their geological settings. We focus on the composition, abundance, and diversity of the macro- and meiofauna associated with the vent mussels. Comparing them between vent fields and among the different edifices of a single vent field (Lucky Strike) allows us to study the influence of exogenous and endogenous factors at local and regional scales. On the local scale, our results show that the pool of species associated with B. azoricus is largely shared between the edifices of LS, which suggest no obstacles to species dispersion across the field. However, differences in relative abundances foster mussel assemblages with varying diversity, probably linked to differences in local abiotic conditions between the various chemistry domains of the field. At larger scale, we observed a "vent field signature" in the communities associated with B. azoricus. Distinct patterns of abundances and diversity were observed across the three fields, with MG appearing as a particular case. The presence of a longitudinal current to the south of this field and its depth difference may partially explain those discrepancies. This study represents one of the most comprehensive integrated faunal and environmental datasets obtained across hydrothermal vent fields