Adaptive strategies of sponges in inland waters

Abstract

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 the realization of an overland dispersal. The gemmule, with its high potentialities as an adaptive device, is the key structure in understanding the life history and the evolution of Spongillidae. The other two families of freshwater sponges, Potamolepidae and Lubomirskidae, do not have a widespread diffusion, probably because of the absence of specialized resistent bodies, such as gemmules, able to act as propagules. In spite of the ability to produce gemmules, few species of spongillids are cosmopolitan; Ephydatia fluviatilis shows the highest plasticity, adapting its life cycle to different climatic conditions: Hibernant and aestivant populations live, respectively, in cold-humid and warmarid climates. The persistent dephasing of the life cycle favours a spéciation trend with the divergence of two morphological subspecies. The production of gemmules is an obligatory phenomenon linked to dispersal and is controlled by endogenous factors. Gem- mulation is a donation process where the ramets, arising from the hatching of a carpet of gemmules, are able to recognize one another and to fuse into a sole functional unit. This fusion is conditioned by the distance between the ramets dispersed by carriers. Theoretically, no difference exists between individuals and populations arising from the same clone. © 1994 Taylor & Francis Group, LLC.