Zooxanthellae diversity in the coral genus Madracis

The coral genus Madracis is probably the second most abundant Caribbean coral. Like all reef-building corals it lives in symbiosis with dinoflagellates. These unicellular algae named zooxanthellae, live as endosymbionts in a wide variety of marine invertebrates. Zooxanthellae were originally believed to be a single pandemic species Symbiodinium microadriaticum. Recent work utilizing genetic data, however, has shown that this is not the case. Zooxanthellae are not only highly diverse they are also found not obligately host-specific. Different zooxanthellae types have been found within a single host species and even within single coral colonies. Restriction Fragment Length Polymorphism (RFLP) analysis of SSrDNA zooxanthellae types in the coral genus Montastrea showed a strong correlation with depth. These results have led to the general view that corals adapt to different photic habitats by harbouring multiple zooxanthellae types. Here we present a similar RFLP analysis of zooxanthellae for another dominant reef building coral, Madracis. Surprisingly, no variation was found in zooxanthellae type either with respect to Madracis morphospecies or depth. This suggest that Madracis has other mechanisms that facilitate adaptation to light and the prevailing generalisation about symbiotic environmental adaptations needs to be re-evaluated.