Diversity, abundance, environmental conditions ...

Three vent fields (Menez Gwen, Lucky Strike, Snake Pit) of the Mid-Atlantic Ridge have been studied during six cruises between 2012 and 2016 on three different research vessels including BICOSE 2014 (Cambon-Bonavita MA ; https://doi.org/10.17600/14000100) on the RV Pourquoi pas?, Momarsat 2012 (Cannat M, Sarradin PM; https://doi.org/10.17600/12040050) on the RV Thalassa, Biobaz 2013 (Lallier F ; https://doi.org/10.17600/13030030), Momarsat 2014 (Cannat M, Sarradin PM, https://doi.org/10.17600/14000300) and Momarsat 2015 (Cannat M, Sarradin PM ; https://doi.org/10.17600/15000200) on the R/V Pourquoi pas? and Momarsat 2016 (Cannat M, Sarradin PM ; https://doi.org/10.17600/16001200) on the RV L'Atalante. During these cruises, the ROV Victor6000 was used to characterize the environmental conditions and collect faunal samples in newly-discovered vent gastropod assemblages. Two distinct assemblages were identified: one dominated by Lepetodrilus atlanticus at the shallowest Menez Gwen vent field and the other by Peltospira smaragdina at the two other investigated fields, Lucky Strike and Snake Pit. The chemistry of 17 gastropod assemblages was characterized prior to faunal sampling using the in situ chemical analyser CHEMINI (Vuillemin et al. 2009) that measures total dissolved sulphide and iron concentrations. In addition, fluid samples within habitats were collected with the PEPITO water sampler (Sarradin et al., 2009). Following habitat characterization, twelve gastropod assemblages were collected using the suction sampler of the submersible from five hydrothermally-active sampling locations at the MG (MG3), LS (Y3, Cypress, Isabel) and SP (Les Ruches) vent fields. The fauna was sorted to the lowest taxonomic level and preserved either in ethanol 96%, formalin 4% or frozen at -80°C for subsequent analyses. Stable isotope ratios of C and N from four key gastropod species (Divia briandi, Lepetodrilus atlanticus, Peltospira smaragdina, Protolira valvatoides) were analysed through continuous flow - elemental analysis - isotope ratio mass spectrometry (CF-EA-IRMS) either at the Laboratoire d’analyse en écologie aquatique et en sédimentologie of Université du Québec à Trois-Rivières (CA) or the LIttoral ENvironnement et Sociétés stable isotope facility at University of La Rochelle (FR).