Latitudinal, longitudinal and bathymetric patterns of abundance, biomass of metazoan meiofauna: importance of the rare taxa and anomalies in the deep Mediterranean Sea

Quantitative information on the spatial distribution of meiofaunal abundance, biomass and biodiversity (as richness of higher taxa) is summarised from 476 sites of the deep- Mediterranean Sea, at depths ranging from ca. 200 to 4617 m. Meiofaunal abundance (46531 and 30624 ind 10 cm2 at 200–1000 and 1000–2000m depth intervals) and biomass (12516 and 11920 mgC 10 cm2 at 200–1000 and 1000–2000m depth intervals) in the bathyal sediments of the Mediterranean Sea are similar to those reported in oceans worldwide but at much higher depths (abyssal or hadal). Meiofaunal abundance, biomass and richness of taxa displayed a common decreasing bathymetric pattern, but showed a steeper negative slope than in other oceanic regions. Latitudinal and longitudinal gradients revealed idiosyncratic patterns when different basins (Western, Central and Eastern) and habitats (open slope, canyon, deep basin and trench) were considered. The results of the non parametric multivariate multiple regression analyses revealed that, conversely to what expected, depth is not the key factor in explaining the variance of meiofaunal assemblages living down to 2000m depth. The quality and quantity of food sources explained a larger fraction of the variance of meiofaunal variables (47, 25 and 33% for abundance, biomass and diversity, respectively) and the importance of food sources increased with increasing depth. However, most of the variance remains unexplained suggesting that other factors (such as episodic events, deep currents, other unexplored yet environmental characteristics) can act a key role in driving the observed meiofaunal spatial patterns. The analysis of rare taxa (on a data set of 183 samples), suggested that differences in the meiofaunal community were evident when dominating taxa (nematodes, copepods and polychaetes) were excluded. We report the presence of rare taxa exclusively present in open slope (amphipods) and canyon (cnidarians and decapod larvae) systems, whereas others are exclusively found in one of the two basins (e.g. gastrotrichs in the Western basin and amphipods and gastropods in the Eastern basin). The apparent preference of some rare taxa for a specific habitat or basin could have important implications for the identification of the hot spots of benthic diversity, for identifying the connectivity among systems and for an appropriate management of deep-sea habitats in the Mediterranean Sea.