2/B – Petrosia - Petrosia ficiformis

Thick, compact and hard (like a stone, from which it gets its name); it has a rough surface, covered in pores and is a purplish red. The coloration is due to the presence of cyanobacteria, which live in the light part of the sponge. Their, number, and therefore also coloration, decrease as does the light. This also influences the shape of the animals, which in caves is elongated and more or less anastomosed. It is easy to meet the nudibranch Peltodoris atromaculata browsing the surface of the sponge.