Their relationship to the coral is considered to be symbiotic rather than parasitic, implying that both species gain from their association, although it is not clear how the coral benefits. Sponges Sponges are uncommon on the shore, but many small species and a few large ones live in shallow water attached to rocks, stones or shells. A simple sponge is a bag of cells surrounding a cavity, supported by microscopic spicules. In more complex species the cavity is folded and there may be many oscules. Minute food particles are filtered out as the water passes through, and gas exchange takes place.
Some sponges are distinctive in shape or colour, but can vary in shape sometimes dramatically.
This makes them very difficult to identify. One common species is a thin encrusting sheet on the tops of boulders exposed to wave action, but becomes much thicker with long slender projections in sheltered places in the same locality.