Biodiversity and zonation pattern of Crustacean Peracarids and Decapods of coastal soft-bottom assemblages (central Tyrrhenian Sea, Italy)

The structure, horizontal and vertical zonations of crustacean peracarid and decapod assemblages were studied from soft bottoms along the Italian coasts of the Tyrrhenian Sea from 42o25’00’’N to 41o12’49’’N. At 69 of the 75 sampled stations, located within three bathymetric ranges (I: 8-15 m; II: 16-27 m; III: 36-53 m), 119 species and 2220 individuals were found. The identified coenotic units were characterized by a constant vertical discontinuity between the second and the third depth-ranges and by a horizontal zonation at the first and the second depth-ranges. The quality of water movements, mainly responsible of the abovementioned vertical discontinuity (Riedl’s second critical depth), and the sedimentation rates (granulometric composition of the sediments towards the depth) act together in structuring assemblages. The nature of surrounding bottoms (rocky shore, rocky banks, seagrasses) and the presence of river outflows, little harbours and/or urban settlements, modifying both the quality of sediment depth-zonation and the trophic conditions, interact with the above-mentioned driving factors in controlling zonation patterns. The resulted heterogeneous distribution of assemblages contributes to increased biodiversity in the studied soft bottoms.