Incongruence between the distribution of a coral reef sponge and phototrophism

Symbiosis between coral reef fauna and microorganisms drives the growth, maintenance and diversity of coral reef habitats. Sponges, a key faunal component of coral reefs, form complex symbiotic relationships with microorganisms which may supply over half of their nutritional requirements through photosynthesis. The habitat distribution of the coral reef sponge Rhopaloeides odorabile correlates with light availability, suggestive of photosynthesis. The present study directly investigated the hypothesis that habitat distribution of R. odorabile is correlated with photosynthesis. Results of photorespirometry trials of 30 ind. exposed to light intensities between 0 and 900 µmol photons m^–2 s^–1 showed no evidence of photosynthesis. Furthermore, no photopigments were present in these sponges and no cyanobacteria could be detected within the tissue. These results did not vary between sponges collected from nutrient rich inner- and mid-shelf reefs, or from oligotrophic outer-shelf reefs. These findings demonstrate that R. odorabile is not a phototrophic sponge and that habitat distributions clearly correlated with light can be due to factors other than photosynthesis.