Competition between the freshwater sponge Ephydatia fluviatilis and the zebra mussel Dreissena polymorpha in Lake Trasimeno (central Italy)

Abstract

The freshwater sponge Ephydatia fluviatilis and the zebra mussel Dreissena polymorpha were found to compete for space in Lake Trasimeno, because they colonize the same hard substrata (rocks and concrete). Photographic monitoring was carried out over one year, from June 2003 to May 2004, at particular sites located along the coasts, namely: Monte del Lago, Passignano sul Trasimeno, Spiaggia Comunale di Castiglione del Lago, Tuoro sul Trasimeno. Sponge overgrowth was observed monthly by comparing images of sponge/mussel interaction. Sponges tend to be encrusting but they can give rise to massive forms on concrete. In both cases, sponges grow and can gradually envelop the valves up to the final encapsulation of the mollusc. Asexual reproduction by means of resistant bodies, or gemmules, allows sponges to withstand the environmental stress, such as desiccation of the habitat or temperature values beyond the limit that are acceptable for mollusc survival. The suppressive influence of E. fluviatilis allows this sponge to be considered a natural enemy of D. polymorpha, acting as a biological control agent on the spreading of the mussel population.