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

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Abstract
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 al., and P. endoumensis Muricy et al., are investigated under light and transmission electron microscopy in a search for new, taxonomically useful characters. Anatomical traits such as ectosome thickness, ectosomal cavities, size of basal cavities, and mesohyl 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 within 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 other 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 between species of Plakina, and the pattern of distribution of bacteria in the mesohyl may help to characterize the genus Plakina within the Plakinidae.