Photosymbiosis in intertidal and subtidal tropical sponges

The frequency of marine sponges that harbour photosynthetic organisms was examined in a tropical area of the Western Indian Ocean. Out of 77 species from five different habitats in Zanzibar, 55 (71.5%) were found to have photosynthetic activity, as assayed by in situ pulse amplitude modulated (PAM) fluorometry (and validated by measurements of chlorophyll content). A significantly higher percentage of the intertidal (85%, n=27) than of the subtidal (64%, n=50) species was found to be photosynthetically active. In addition, in situ measurements of a common intertidal Haliclona sp. confirmed that photosynthetic activity was maintained during prolonged periods of air exposure at low tide. The fact that each habitat contained its own characteristic species (only eight species were found in more than one habitat) suggests that the generally high presence of photosymbionts in all the various intertidal communities may be an important component to the successful adaptation of those species to life in the intertidal. We propose two possible explanations for the higher presence of photosymbiotic species in the intertidal: 1) Intertidal sponges may be more dependent on autotrophic symbionts to meet their energetic needs because they are unable to filter-feed during air exposure at low tide; 2) The photosynthetic symbionts provide UV protection in an environment where UV radiation is high.