Deep‐sea nematode biodiversity in the Mediterranean basin: testing for longitudinal, bathymetric and energetic gradients

The knowledge of the processes controlling the spatial distribution of species diversity is one of the main challenges of the present ecological research. Spatial patterns of benthic biodiversity in the deep sea are poorly known in comparison with other ecosystems and this limits our understanding of the mechanisms controlling the distribution and maintenance of high biodiversity in the largest ecosystems of our biosphere. Although the Mediterranean basin covers <1% of the world ocean surface, none the less it hosts >7.5% of the global biodiversity. The high biogeographic complexity and the presence of steep ecological gradients contribute in making the Mediterranean a region of very high diversity. Here we report the results of an investigation on the patterns of nematode biodiversity in the deep‐Mediterranean Sea, in relation with bathymetric, longitudinal and energetic gradients. Our results indicate that benthic biodiversity in the deep‐Mediterranean decreases significantly with increasing depth. Moreover, at equally deep sites, nematode diversity decreased from the western to the eastern basin and longitudinal gradients were evident when comparing sites at 4000‐m depth, with 3000‐m depth. The analysis of the available energy (measured as labile organic matter content of the sediments) suggests that biodiversity patterns are not controlled by the amounts of food resources, but instead bio‐availability is the key factor. A more detailed analysis revealed an extremely high deep‐sea beta‐diversity (turnover diversity), both among sites at different depths as well as at similar depths of different longitude or within the same basin. This new finding has not only important implications on the estimates of the overall regional diversity (gamma diversity), but also suggests the presence of high biogeographic complexity in the deep benthic domain of the Mediterranean Sea.