The snails' tale in deep-sea habitats in the Gulf of Cadiz

Bridging the Atlantic and Mediterranean conti- nental margins, the South Iberian region has recently been the focus for geological and biological investigations. In this region, the Gulf of Cadiz (GoC) encompasses a great vari- ety of deep-sea habitats that harbour highly diverse biologi- cal communities. In this study, we describe the composition of gastropod assemblages obtained from in situ colonization experiments and benthic sampling of deep-sea habitats in the GoC. Gastropod distributional patterns, such as bathy- metric ranges, bathymetric turnover, affinity to substrate types and abundance-occupancy relationships, are analysed and interpreted in relation to their inferred dispersal capa- bilities and substrate availability. Overall, the GoC com- prises a high diversity of gastropods (65 species), and dis- tinct assemblages were found in typical sedimentary envi- ronments at mud volcanoes and in association with carbon- ate and coral samples or organic substrata. The number of taxa peaked at the Carbonate Province in the middle slope (600-1200 m depth), a highly heterogeneous area with nu- merous mud volcanoes, carbonate mounds and corals. Dar- win (1100 m) and Captain Arutyunov (1300 m) mud vol- canoes harboured the most species-rich and abundant gas- tropod assemblages, respectively. Colonization experiments with organic substrata (wood and alfalfa grass) also yielded diverse and abundant gastropod assemblages. These organic inputs allowed the recruitment of local species but mainly of wood specialist taxa that were not previously known from the GoC. Our results suggest that the distribution of gastro- pod assemblages may be primarily determined by the oc- currence of suitable habitats, probably due to the effect of the substrate type on the structural complexity of the habitat and availability and diversity of adequate food sources. The type of larval development is apparently not a limiting fac- tor for colonization of deep-sea habitats. However, the pre- dominance of non-planktotrophy, and especially lecithotro- phy, suggests that a trade-off between more limited disper- sal capability and higher potential for self-recruitment may be a recurrent pattern in gastropod species inhabiting re- ducing environments and other patchily distributed deep-sea habitats. A network of suitable habitats that ensures effec- tive population connectivity would explain the predominance and relatively wide distribution of short-distance dispersing non-planktotrophic species in the GoC deep-sea habitats and other geographical regions.