Epibiontic and endobiontic polychaetes of Geodia cydonium (Porifera, Demospongiae) from the Mediterranean Sea

Polychaete assemblages associated to the sponge Geodia cydonium were invest igated at two sampling sites in the Mediterranean Sea: Porto Cesareo Basin (Apulia) and Marsala Lagoon (Sicily), both characterized by sheltered hydro dynamic conditions. Samples were seasonally performed during 1997, in order to compare the assemblages coming from the two localities studied, conside ring separately the internal and external tissues of the sponge, and with t he aim of evaluating the influence of sponge size on polychaete colonizatio n. The examined sponge is characterized by a peculiar stratification of its tissues: an external thick and hard layer, the cortex, and an internal sof ter one, the choanosome. Statistical analysis showed that this was the main factor controlling polychaete assemblage, with the internal tissue, less r ich and diversified, appearing impoverished with respect to the external la yer. A similarity in species composition was observed between sites, even t hough some differences were evidenced in the abundance of some species, mai nly reflecting differences in local environmental conditions. Species richn ess and density increased with the increasing sponge size. Such a situation is particularly evident at Porto Cesareo, where sponges are covered by an algal layer which is particularly rich on the largest specimens, thus sugge sting that most of the species of polychaetes were linked more to the neigh bouring environment than to the sponge itself.