Mollusc assemblages of hard bottom subtidal fringe: a comparison between two coastal typologies

The mollusc assemblages of subtidal fringe from two different coastal typologies are described in their qualitative and quantitative features. The large-scale spatial investigation has been carried out in the lava cliffs of Catania and the conglomerate “beach-rocks” of Capo Peloro (Messina), whose assemblages have been compared by fourteen shallow sampling stations, spaced out hundred/thousand meters apart. The similarity/dissimilarity levels of the two assemblages have been evaluated throughout a set of eighty-six species, exclusive or common between the two areas. Both the assemblages were characteristic of an impoverished and highly variable photophilic taxocoenosis. The area was the main discriminating factor that determined the highest richness and abundance in the rough lava surface. The Catania assemblage was more constant in species composition, with presence of exclusive bivalves, cue of a micro-sedimentary environment. The Messina assemblage was very variable in species composition, and its structure, dominated by motile gastropods, was evidence of a high energy environment. Differences in the structure and micro-topography of the natural substratum from the two areas, besides possible secondary influence of freshwater inputs and wave exposure, were factors mainly responsible for the observed patterns. The whole data set, with dominant and accessory taxa, involves a relevant contribution from the deeper subtidal assemblage; despite of their ephemeral character, these assemblages contribute to maintain the local biodiversity on a broader spatial scale.