ABSTRACT
Studies of epibiotic fauna on Pinna nobilis and Pinna pernula from the Straits of Messina have shown a considerable incidence of the rare echinoid Arbaciella elegans. The observation that this species has not found by us in other hard bottoms, in the same environment, suggest that, at least in the Straits of Messina, the valve surface of Pinna could be an elective habitat for the species. Probably the shell marked ornamentation and the implant position into the substratum, satisfy the cryptic habitus and tha peculiar trophism of Arbaciella. Considerations about hydrodynamic features of the Messina Straits lead us to think that good vivificated waters and an intensive transport of organic matter are the most important limiting factors for the A. elegans distribution.
