A case study of mollusc and polychaete soft-bottom assemblages submitted to sedimentary instability in the Mediterranean Sea.

In the Strait of Messina (Sicily, Central Mediterranean Sea) strong tidal currents, very irregular hydrological regime and related solid load, and local seismic activity cause sediment instability; this area represents therefore a case study of a naturally disturbed soft-bottom environment. In this paper, mollusk and polychaete assemblages of the northern part of the Strait were described. Composition, community structure, eco-ethological features, trophic guilds and sediment-type relationships were studied in 64 stations sampled located between 3.5 and 50 m depth in October 1992 by means of a Van Veen grab. A total of 131 species (65 molluscs and 66 polychaetes) were identified. A dense population of the tubicolous polychaete Ditrupa arietina was recognized, together with the occurrence of other species tied to a high sedimentation rate, such as Corbula gibba and Tellina distorta, as well as widespread, mud-tolerant species (e.g. Chone spp., Hyalinoecia tubicola). Diversity (H¢) showed a peak at intermediate depths (10/20 m) and a clear decrease beyond this depth, corresponding to the Ditrupa core population. Multivariate comparison between sediment features and community composition throughout the bathymetric gradient showed a narrow ecocline between two environments subjected to opposite hydrodynamic constraints. In the shallower zone, a wide typology of trophic-ethological guilds was related to community patchiness, in contrast to a greater functional uniformity of the deeper assemblage, dominated by sessile, semi-infaunal suspension feeders. A possible role played by a phase of increased rainfall to increase bottom instability, locally emphasized by a previous human activity, is also discussed.