Sponge: effect on the form of reef corals.

The sponge, Mycale laevis, when encrusting the lower surfaces of flattened reef corals, induces marked peripheral folding of the host colonies. This relationship, though facultative, has advantages for both associates. The sponge has a continually enlarging substrate that is free from competitive sessile forms. The coral may benefit from an increased feeding efficiency as a result of water currents produced by the sponge and it is protected from invasion by boring forms, notably clionid sponges.