Sponges are essentially heterotrophic organisms, feeding on pelagic primary producing micro-organisms, on heterotrophic bacteria or organic molecules in the open water column. It has increasingly been realized that some sponges contain large quantities of symbiotic green bacteria which contribute considerably to the food of their hosts, in a way comparable to that of zooxanthellae and their coral hosts. Although symbiotic green bacteria occur in sponges all over the world, the Indo-West Pacific harbours certain genera that have adapted to this symbiosis to such an extent, that their net production: respiration rate is positive. They often have a characteristic thin-bladed form. The distribution of phototrophic sponges over de Great Barrier Reef has been studied by Wilkinson c.s. (e.g. 1989), and he found that they are concentrated in the most oligotrophic oceanic parts of the GBR, whereas they are rare or absent in the inner nearshore reefs. In a parallel study, Van Soest (1993) demonstrated that in the Seychelles phototrophic sponges are confined to low carbonate islands with no or very little terrestrial run-off, whereas they are absent from the high granitic islands with considerable terrestrial run-off. These independent observations prove that phototrophy in sponges is an adaptation to oligotrophy.

Source: Rob van Soest