Only three species of the sponge genus Plakina Schulze have been described from the Mediterranean since 1880, in spite of a large amount of allegedly intraspecific variability in morphological characters. However, recent genetic studies based on electrophoretic techniques have revealed extensive cryptic speciation in north-western Mediterranean Plakina, demonstrating that most of this variation was interspecific rather than intraspecific. We describe in
detail the morphology and anatomy of four new species of Plakina from the Mediterranean—P.crypta, P. weinbergi, P. endoumensis and P. jani—of which the latter two were discovered through allozyme electrophoresis. Plakina monolopha Schulze and P. trilopha Schulze
are redescribed, and their morphological and geographical limits are discussed along with those of P. dilopha Schulze. Accurate analysis of the internal anatomy and of the shape and ramification pattern of lophose spicules in scanning electron microscopy provides new,
powerful morphological criteria for species discrimination in Plakina. More widespread use of such new taxonomic characters should provide evidence against the alleged cosmopolitanism
of some Plakina species, thus generating an increase in estimates of the biodiversity of plakinids.
