Barrel sponge

Spongia officinalis, the commercial bath sponge, was historically harvested to near-local extinction across the Aegean but has partially recovered at deep undisturbed sites like Prasonisi. Individual specimens at 25 to 35 m exceed 60 cm in diameter and may be several decades old, filtering up to 10,000 litres of seawater per day and providing den space for shrimp, nudibranchs, and small gobies within their oscules. The species is a sensitive indicator of water quality — its return to sites like Prasonisi correlates directly with improved coastal water management on southern Rhodes.