The molluscan assemblages inhabiting the leaves and rhizomes of a deep water Posidonia oceanica settlement in the central Tyrrhenian Sea

The molluscan assemblages inhabiting the leaf and rhizome layers of Posidonia oceanica were studied in a deep water (–24/26 m) settlement of a highly heterogeneous substratum on an off-shore reef in the central Tyrrhenian Sea. This is one of the few works dealing with the rhizome layer and with Posidonia oceanica that has settled on hard substrata. The leaf assemblage only had a few species, less than other assemblages at a comparable depth in the same basin. This poorness may be due to the depth, but it may also be due to the high fragmentation of the meadow. However, the high percentage of carnivores is consistent with previous observations in deep water meadows. The rhizome assemblage is highly diverse both in terms of species and feeding guilds, which could be explained by the higher affinity for low light conditions of most molluscs and the greater habitat heterogeneity. The marked differences in the two taxocoenoses and the high diversification of the rhizome assemblage evidence that they should be included in studies on the potential diversity of Posidonia oceanica meadows. Failure to consider this layer seriously affects any evaluation of the biodiversity of this habitat, which is of great conservation interest.