Deep-sea ecosystems represent the largest biome of the global biosphere, but
knowledge of their biodiversity is still scant. The Mediterranean basin has been
proposed as a hot spot of terrestrial and coastal marine biodiversity but has
been supposed to be impoverished of deep-sea species richness. We summarized all
available information on benthic biodiversity (Prokaryotes, Foraminifera,
Meiofauna, Macrofauna, and Megafauna) in different deep-sea ecosystems of the
Mediterranean Sea (200 to more than 4,000 m depth), including open slopes, deep
basins, canyons, cold seeps, seamounts, deep-water corals and deep-hypersaline
anoxic basins and analyzed overall longitudinal and bathymetric patterns. We
show that in contrast to what was expected from the sharp decrease in organic
carbon fluxes and reduced faunal abundance, the deep-sea biodiversity of both
the eastern and the western basins of the Mediterranean Sea is similarly high.
All of the biodiversity components, except Bacteria and Archaea, displayed a
decreasing pattern with increasing water depth, but to a different extent for
each component. Unlike patterns observed for faunal abundance, highest negative
values of the slopes of the biodiversity patterns were observed for Meiofauna,
followed by Macrofauna and Megafauna. Comparison of the biodiversity associated
with open slopes, deep basins, canyons, and deep-water corals showed that the
deep basins were the least diverse. Rarefaction curves allowed us to estimate
the expected number of species for each benthic component in different
bathymetric ranges. A large fraction of exclusive species was associated with
each specific habitat or ecosystem. Thus, each deep-sea ecosystem contributes
significantly to overall biodiversity. From theoretical extrapolations we
estimate that the overall deep-sea Mediterranean biodiversity (excluding
prokaryotes) reaches approximately 2805 species of which about 66% is
still undiscovered. Among the biotic components investigated (Prokaryotes
excluded), most of the unknown species are within the phylum Nematoda, followed
by Foraminifera, but an important fraction of macrofaunal and megafaunal species
also remains unknown. Data reported here provide new insights into the patterns
of biodiversity in the deep-sea Mediterranean and new clues for future
investigations aimed at identifying the factors controlling and threatening
deep-sea biodiversity.
