BIORHYTHM AND ENVIRONMENTAL-CONTROL IN THE LIFE-HISTORY OF EPHYDATIA-FLUVIATILIS (DEMOSPONGIAE, SPONGILLIDAE)

The life cycle of Ephydatia fluviatilis involves obligatory gemmule fo rmation, a process strongly related to environmental conditions. A com parative investigation carried out on three populations of this specie s distributed along a climatic gradient, besides confirming a link bet ween occurrence of gemmules and the severity of the habitat, evidences that gemmulation may be triggered even by endogenous stimuli. Indeed, in the northern area (Torrente Scrivia, Liguria), subjected to ice-up and notable thermic and water-flow fluctuations, the sponges hibernat e for a long period and show shorter summer activity. In the southern area (Canale Tagliavia, Sicily), with a summer dry-up, the active spon ges are present during the winter, while the quiescence coincides with an aestivation. The persistence of this rhythm over the years is disc ussed, also in consideration of a speciation trend which points to a p hysiological reproductive barrier between these metapopulations. By co ntrast, in the intermediate area (Rio Bunnari, Sardinia), notwithstand ing the lack of a summer dry-up, the population shows a short summer q uiescence. This life cycle, not depending on environmental factors, su pports the hypothesis of an intrinsic rhythm inducing the quiescence o f E. fluviatilis.