Marine gastrotrichs from west-Mediterranean calcareous sediment

In the western Mediterranean Sea conspicuous patches of calcareous soft bottoms are quite rare, one exception being the surroundings of Pianosa, a small Italian island, that for long served as a State high security penitentiary. In 1998 the island changed its status, becoming part of the Tuscan Archipelago National Park, thus allowing in small contingents of visitors and researchers. This novelty gave us the opportunity to survey the gastrotrich community of calcareous habitats, whose knowledge outside the tropical coralline realm is scarce. Thus, in July 2003, we sampled qualitatively more than 15 sites in front of the north-eastern coast of the island. Samples consisting of either coarse biogenic or medium clastic sand were collected manually by SCUBA diving at depths between -1.5 m and –16.5 m isobaths. Additionally, we collected quantitative samples, in four replicates, from three sites along a transect, at 1.5 m, 5.0 m and 12 m water depth respectively. Faunistic analysis revealed 40 species from the eighteen sites explored, yielding a total of 146 records (species x site) or 8.1  4.7 species per site. Twenty-six species in twelve genera and five families belong to the order Macrodasyida, and fourteen species in five genera and two families belong to the order Chaetonotida. All but four appear undescribed species, with two endemic to the island, so far. Particularly interesting is the simultaneous presence of the three described species of the uncommon genus Lepidodasys. Higher species richness resulted in sites characterised by medium, clastic sediments compared to the coarse, biogenic ones, 11  3.8 spp vs 4.5  2.6 spp. Density was higher in the finer sediment too, 211.9  115.0 ind.·10 cm-2 vs 6.7  4.6 ÷ 9.3  2.9 ind.·10 cm-2. Gastrotrichs ranked third in abundance among meiofauna taxa in sites of medium granulometry, making up 5.5% of total meiofauna, whereas they resulted only seventh or lower in sites characterized by coarse sand, accounting just for 0.9-1.2% of the total multicellular meiobenthos. As a working hypothesis we link the generally poor gastrotrich fauna found in the coarser sediments to the limited extension of the sandy patches from which the samples were taken e.g., small areas in Posidonia meadow. Preliminary comparisons indicate that clastic calcareous sediments of medium granulometry host a gastrotrich community richer then siliceous sand of corresponding grain size. Studies on a broader scale indicate that this difference in biodiversity in the different sediment types can be generalized (see contribution by Giere et al.), clarification of the reasons is the aim of a special project.