Necrosis in a population of Petrosia ficiformis (Porifera, Demospongiae) in relation with environmental stress

Healthy specimens of the Mediterranean Petrosia ficiformis harbour endocell ular cyanobacteria (Aphanocapsa feldmanni) causing a violet pigmentation of the sponge. Necrosis in P. ficiformis can be easily detected by the occurr ence of white patches scattered over the surface. Necrotic specimens were e xamined along the Gallinara Island coasts (Western Ligurian Sea), in coinci dence with environmental stress (heavy rainfall, land run-off, high sea-wat er temperature). The appearance of white patches is due to the gradual slou ghing of the pinacodermal covering, as evidenced by scanning electron micro scopic observations Sloughing leads to progressive tissue degeneration in t he deeper parts. Histological sections showed that, concomitantly with the loss of the superficial layer, internal sponge tissues degenerate and the s ponge body becomes exposed to the invasion of ciliates. Spicule bundles of the skeletal network separate damaged tissues from the healthy ones, thereb y slowing down spread of necrosis and enabling successful recovery.