ABSTRACT
Samplings carried out by dredge on hard circalittoral bottoms of the Messina Straits, have shown the presence of great populations of the ophiuroid Ophiactis balli at 80 to 200 meters of depth. O. balli is localized in the narrow sill limiting the northern part of the Straits. The sill is characterized by rough bottoms with pinnacles. The hydrodynamic regime bound to the tides is very intense and interested by levantine intermediate waters rising, particularly in the western part of the sill (De Domenico, 1987). This environment, recently identified as a particular facies of the off shore rockies biocoenosis (RL, Pérès & Picard, 1964), is characterized by a cospicuous epibiotic fauna, dominated by the Stylasteridae Errina aspera and the cirripede Pachylasma giganteum (Di Geronimo & Giacobbe, 1987; Di Geronimo & Fredj, 1987). Rich communities of O. balli live on the bases of P. giganteum and E. aspera, in the substratum cavities made up by Corallinacee. In this community, characterized by oligospecificity and absolute dominance of suspension-feeders, the hydrodynamism represents both the most important limiting factor and the main organic matter carrier. The interspecific competition is regulated by a distinct populations stratifications, where the more reophile and microphage species live on a substratum populated by a less specialized cryptic fauna, which benefit by the hydrodynamism local decrease. We can remember, in this respect, that in the Ophiactis genus morphological and functional adaptations to the strong and alternate currents are known (Lawrence, 1987). These adaptations, which regard essentially feeding mechanism, arranges for the species to result deprived of competitors and so abundant to justify the definition of facies. For this reasons the abundance of this ophiuroid, that elsewhere is rare and localized could be explained (Tortonese, 1965). Finally from these considerations O. balli could be one of the “atlantic” elements which peculiarly characterize the Messina Straits.
