The snails' tale at deep-sea habitats in the Gulf of Cadiz (NE Atlantic)

Bridging the Atlantic and Mediterranean continental margins, the South Iberian region has recently been the focus for geological and biological investigations. The Gulf of Cadiz (GoC) encompasses a great variety of deep-sea habitats that harbour highly diverse biological communities. In this study, we describe the taxa composition of gastropod assemblages from deep-sea habitats in the GoC and analyse the species distributional patterns in relation to their dispersal capabilities and substrate availability. Distinct gastropod assemblages were found at mud volcanoes, carbonate and coral sites, and organic-falls. Overall, the GoC comprises a high diversity of gastropods that include 65 taxa representing 32 families, 48 genera and 30 named species. The highest number of taxa was found at the highly heterogeneous carbonate province in the middle slope (500–1500 m depth), and higher abundance of individuals was observed in Captain Arutyunov mud volcano, one of the most active sites found in the GoC. Faunal similarities were found with Mediterreanean cold-seeps (species- and genus-level) and other chemosynthetic environments in the Atlantic and Pacific Oceans (genus-level). Colonization experiments with organic substrata (wood and alfalfa grass) yielded high abundances of gastropod species. These organic inputs allowed the recruitment of local species but also of wood specialist taxa that were not known to occur in the GoC. Our results suggest that distribution of gastropod assemblages may be primarily determined by the occurrence of suitable habitats probably due to effect of the substrate type on feeding strategies and that larval development is not a limiting factor for colonization of the deep sea. However, the predominance of non-planktotrophy, and especially lecithotrophy, suggests that the trade-off between a more limited dispersal capability and the higher potential for self-recruitment may be favoured by the gastropod species inhabiting reducing environments and other patchily distributed deep-sea habitats. A network of suitable habitats that ensures connectivity of effective populations would explain the predominance and relatively wide distribution of short-distance dispersing non-planktotrophic species at the GoC deep-sea habitats and other geographical regions.