ADAPTIVE STRATEGIES OF SPONGES IN INLAND WATERS

The notable plasticity of Porifera allowed the colonization of inland waters during the Mesozoic Era as a consequence of the gradual morpho- functional adaptations of dormant bodies, i.e., the appearance of the pneumatic coat and of spiny spicules, which could be responsible for t he realization of an overland dispersal. The gemmule, with its high po tentialities as an adaptive device, is the key structure in understand ing the life history and the evolution of Spongillidae. The other two families of freshwater sponges, Potamolepidae and Lubomirskidae, do no t have a widespread diffusion, probably because of the absence of spec ialized resistent bodies, such as gemmules, able to act as propagules. In spite of the ability to produce gemmules, few species of spongilli ds are cosmopolitan; Ephydatia fluviatilis shows the highest plasticit y, adapting its life cycle to different climatic conditions: hibernant and aestivant populations live, respectively, in cold-humid and warm- arid climates. The persistent dephasing of the life cycle favours a sp eciation trend with the divergence of two morphological subspecies. Th e production of gemmules is an obligatory phenomenon linked to dispers al and is controlled by endogenous factors. Gemmulation is a clonation process where the ramets, arising from the hatching of a carpet of ge mmules, are able to recognize one another and to fuse into a sole func tional unit. This fusion is conditioned by the distance between the ra mets dispersed by carriers. Theoretically, no difference exists betwee n individuals and populations arising from the same clone.