Flat Sponge

Photo: Dean Zagorac · CC BY-NC

Petrosia ficiformis is the dominant encrusting organism in Žanjice's deeper sandy zones, forming irregular plates up to half a metre across in shades from grey to violet. These sponges are long-lived habitat engineers that provide shelter for brittlestars, amphipods, and juvenile fish beneath their raised edges. They are also the primary food source for several nudibranch species and the chemical compounds they produce are the subject of active pharmaceutical research.