BIODIVERSITY RESEARCH Pockmarks enhance deep-sea benthic biodiversity: a case study in the western Mediterranean Sea Daniela Zeppilli 1 , Miquel Canals 2 and Roberto Danovaro 1 * INTRODUCTION The Mediterranean Basin is recognized as a terrestrial biodi- versity hotspot with a uniquely high percentage of endemic species (Myers et al., 2000). This applies also to the marine counterpart, as despite its relatively small dimension (0.82% of the ocean surface and ca. 0.3% of the global ocean volume), the Mediterranean Sea hosts approximately 17,000 species (Coll et al., 2010), which corresponds to approximately 7% of the global marine biodiversity. However, it has been assumed for a long time that the deep Mediterranean Sea has very low biodiversity, especially when compared to other deep-ocean regions (Danovaro et al., 2010). While it has been estimated that 66% of the deep-sea species are still unknown to science, 1 Department of Life and Environmental Sciences; Polytechnic University of Marche, Via Brecce Bianche, 60131 Ancona, Italy, 2 GRC Geocie`ncies Marines, Departament d’Estratigrafia, P. i Geocie`ncies Marines, Facultat de Geologia, Universitat de Barcelona, C/Martı ´i Franque`s s/n, Campus de Pedralbes, E-08028 Barcelona, Spain *Correspondence: Roberto Danovaro, Department of Life and Environmental Sciences; Polytechnic University of Marche, Via Brecce Bianche, 60131 Ancona, Italy. E-mail: r.danovaro@univpm.it ABSTRACT Aim Pockmarks are craters on the sea floor formed by sub-sea-floor fluid expulsions, which occur world-wide at all ocean depths. These habitats potentially host a highly specialized fauna that can exploit the hydrocarbons released. Pockmarks at relatively shallow depths can be easily destroyed by human activities, such as bottom trawling. In the present study, we investigated the combined effects of sea-floor heterogeneity, rate of fluid emission and trophic conditions of different pockmarks on the biodiversity of the deep-sea assemblages. Location Continental slope of the Gulf of Lions, western Mediterranean Sea, at water depths from 265 to 434 m. Methods We investigated the biodiversity associated with sea-floor pockmarks that are both inactive and that have active gas emissions. Control sites were selected on the sea floor outside the influence of the gas seepage, both within and outside the pockmark fields. We examined the combined effects of: (i) sea-floor heterogeneity; (ii) variable levels of fluid (gas) emissions; and (iii) trophic characteristics of the meiofaunal assemblage structure and nematode diversity. Results Sediments within the pockmark fields had lower meiofaunal abundance and biomass when compared with the surrounding sediments that were not influenced by the gas seepage. Although several higher taxa were absent in the pockmarks (e.g. Turbellaria, Tardigrada, Cumacea, Isopoda, Tanaidacea, Nemertina and Priapulida, which were present in the control areas), the richness of the nematode species within all of these pockmarks was very high. About 25% of the total species encountered in the deep-sea sediments of the investigated areas was exclusively associated with these pockmarks. Main conclusions We conclude that both active and inactive pockmarks provide significant contributions to the regional (gamma) diversity of the continental slope in the western Mediterranean Sea, and thus the protection of these special and fragile habitats is highly relevant to the conservation of deep-sea biodiversity. Keywords Deep-sea biodiversity, gas seepage, meiofauna, nematodes, pockmarks. Diversity and Distributions, (Diversity Distrib.) (2012) 18, 832–846 DOI:10.1111/j.1472-4642.2011.00859.x 832 http://wileyonlinelibrary.com/journal/ddi ª 2011 Blackwell Publishing Ltd A Journal of Conservation Biogeography Diversity and Distributions