Nematode diversity patterns at different spatial scales in bathyal sediments of the Mediterranean Sea

Understanding biodiversity patterns and how they are driven at different spatial scales is a crucial issue in ecological studies. This is particularly evident for the deep sea, the largest biome of the biosphere, where information on the scales of spatial variation is very scant. Here, we investigated deep-sea nematodes species richness, turnover and functional diversity, and life strategies at different spatial scales (from local to macro-regional) to identify the factors that shape regional (gamma) and macro-regional (epsilon) deep-sea diversity. This study was conducted in several deep-sea habitats (canyons, open slopes, deep-water corals, and bathyal plains) over > 2000 km across the whole Mediterranean Basin, at a bathymetric range comprised between ca. 600 and 1300 m. Our results indicate that the patterns of local (alpha) diversity across the deep Mediterranean follow the gradients of the trophic conditions, which decrease from the western to the eastern basins. For all of the sites and habitats, the alpha diversity is generally low. Conversely, the turnover diversity changes significantly among habitats (beta diversity) and between regions (delta diversity), showing values of dissimilarity (based on species presence/absence matrixes) between 59 and 90% for beta diversity and between 81 and 89% for delta diversity. This suggests that patterns and values of gamma and epsilon diversities in the deep Mediterranean Sea are related to turnover diversity among habitats and between regions (beta and delta diversities), rather than to the local biodiversity (alpha diversity). These results indicate also that the differences in beta and delta diversities are even more important than those in alpha diversity for the comprehension of the drivers of biodiversity in the deep Mediterranean Sea. We conclude that the presence of different habitats and gradients in environmental conditions, by promoting a high turnover diversity across the Mediterranean Sea, may play a crucial role in the levels of gamma diversity of deep-sea nematodes.