Anatomy, cytology and symbiotic bacteria of four Mediterranean species of Plakina Schulze, 1880 (Demospongiae, Homosclerophorida)

The internal anatomy, cytology and symbiotic bacteria of four Mediterranean species of the sponge genus Plakina Schulze, namely P, trilopha Schulze, P . jani Muricy et al., P. crypta Muricy et nl., and P. endoumensis Muricy ed al., are investigated under light and transmission electron microscopy in a search for new, taxonomically useful characters. Anatomical traits such a s ectosome thickness, ectosomal cavities, size of basal cavities, and mesoh yl:chambers ratio can be useful to distinguish groups of species within the genus, but give no clear indication of relationships of Plakina with other plakinid genera. The most useful cytological traits at the species level w ithin Mediterranean Plakina are the presence/absence of vacuolar cells and of pseudopodia in pinacocytes, and the number of microvilli in choanocytes. There are no cells with inclusions in the mesohyl, and the simple cytology of Plakina agrees more closely with that of Corticium candelabrum Schmidt, Oscarella lobularis Schmidt and O. tuberculata (Schmidt) than to that of o ther species of Oscarella and Pseudocorticium jarrei Boury-Esnault et al. A unique combination of symbiotic bacteria is associated with each species. The number and composition of bacterial types can be used to discriminate b etween species of Plakina, and the pattern of distribution of bacteria in t he mesohyl may help to characterize the genus Plakina within the Plakinidae .