THE LIFE OF A SPONGE IN A SANDY LAGOON

Infaunal soft-bottom invertebrates benefit from the presence of sedime nt, but sedimentation is potentially harmful for hard-bottom dwellers. Most sponges live on hard bottom, but on coral reefs in the Red Sea, the species Biemna ehrenbergi (Keller, 1889) is found exclusively in s oft-bottom lagoons, usually in the shallowest part. This location is a sink environment, which increases the deposition of particulate organ ic matter. Most of the sponge body is covered by sediment, but the chi mney-like siphons protrude from the sediment surface. The sponge is at tached to the buried beach-rock, which reduces the risk of dislodgment during storms. Dye injected above and into the sediment revealed, for the first time, a sponge pumping interstitial water (rich with partic les and nutrients) into its aquiferous system. Visual examination of p lastic replicas of the aquiferous system and electron microscopical an alysis of sponge tissue revealed that the transcellular ostia are most ly located on the buried surface of the sponge. The oscula, however, a re located on top of the siphons; their elevated position and their ab ility to close combine to prevent the filtering system outflow from cl ogging. The transcellular ostia presumably remain open due to cellular mobility. The sponge maintains a large population of bacteriocytes, w hich contains bacteria of several different species. Some of these bac teria disintegrate, and may be consumed by the sponge.