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

Healthy specimens of the Mediterranean Petrosia ficiformis harbour endocellular cyanobacteria (Aphanocapsa feldmanni) causing a violet pigmentation of the sponge. Necrosis in P. ficiformis can be easily detected by the occurrence of white patches scattered over the surface. Necrotic specimens were examined along the Gallinara Island coasts (Western Ligurian Sea), in coincidence with environmental stress (heavy rainfall, land run-off, high sea-water temperature). The appearance of white patches is due to the gradual sloughing of the pinacodermal covering, as evidenced by scanning electron microscopic observations Sloughing leads to progressive tissue degeneration in the deeper parts. Histological sections showed that, concomitantly with the loss of the superficial layer, internal sponge tissues degenerate and the sponge body becomes exposed to the invasion of ciliates. Spicule bundles of the skeletal network separate damaged tissues from the healthy ones, thereby slowing down spread of necrosis and enabling successful recovery.