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 B1% 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 bioavailability
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.