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

Abstract

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.

Deep, diverse and definitely different: unique attributes of the world's largest ecosystem

“…Although the distribution of nematode genera across different deep-sea environments is rather uniform (Thistle and Sherman, 1985;Vincx et al, 1994;, there is little overlap of nematode species composition between adjacent sites (Jensen, 1988;Tietjen, 1989). Knowledge of nematode species is limited to the Mediterranean (Vitiello, 1976;Danovaro et al, 2008b), the North Atlantic (Tietjen, 1976;Dinet and Vivier, 1979;Jensen, 1988), the Clarion-Clipperton fracture zone (Lambshead et al, 2003;Miljutina et al, 2009) and the Atacama slope and trench (Gambi et al, 2003). Moreover, most of these studies use species identifications only for biodiversity measurements without providing information on species composition and turnover.…”

“…From 200 m depth downwards, specific genera appear or become more abundant compared to shallow water sites along shelves. Some typical deep-sea genera include Acantholaimus, Thalassomonhystera and Halalaimus, the first two rarely recorded in shallow waters but all three present with relatively high numbers from 200 m down to the deep trenches all over the world Gambi et al, 2003;Danovaro et al, 2008bDanovaro et al, , 2009). …”

“…A variety of unanticipated oceanographic conditions, at specific depths, often interrupts and modifies bathymetric horizontal diversity trends Levin and Gage, 1998;Vetter and Dayton, 1999;Levin et al, 2001). Species diversity varies strongly with depth and diversity-depth patterns vary geographically from basin to basin (Rex et al, 1997;Danovaro et al, 2008b). Although diversity-depth trends are not completely understood, it seems likely that they are shaped by complex interacting factors that operate at different temporal and spatial scales (Levin et al, 2001;Stuart et al, 2003).…”

From microbes to macrofauna: an integrated study of deep benthic communities and their response to environmental variables along the Malta Escarpment (Ionian Sea)

“…The high proportion of deposit-feeding nematodes in the deep sea reflects the important role of bacteria in their nutrition (Danovaro et al 2008b). Furthermore, the dominance of large detritivores and deposit-feeding nematodes indicates that they play important roles in the carbon recycling in the benthic food web.…”

Analysis of the community structure of abyssal kinetoplastids revealed similar communities at larger spatial scales

“…Danovaro et al (2008a) reported a clear link between benthic diversity and ecosystem functioning. They estimated that a biodiversity loss of 20-30% could result in a 50-80% reduction of deep-sea ecosystems' key processes (Danovaro et al, 2008b). Knowledge of the spatial dimension of diversity and hence the need for large-scale community sequencing is necessary to evaluate the mechanisms driving biodiversity and biogeography in this vast but poorly understood ecosystem (Azam and Worden, 2004).…”