The Symbiotic Anthozoan: A Physiological Chimera between Alga and Animal

The symbiotic life style involves mutual ecological, physiological, structural, and molecular adaptations between the partners. In the symbiotic association between anthozoans and photosynthetic dinoflagellates (Symbiodinium spp., also called zooxanthellae), the presence of the endosymbiont in the animal cells has constrained the host in several ways. It adopts behaviors that optimize photosynthesis of the zooxanthellae. The animal partner has had to evolve the ability to absorb and concentrate dissolved inorganic carbon from seawater in order to supply the symbiont's photosynthesis. Exposing itself to sunlight to illuminate its symbionts sufficiently also subjects the host to damaging solar ultraviolet radiation. Protection against this is provided by biochemical sunscreens, including mycosporine-like amino acids, themselves produced by the symbiont and translocated to the host. Moreover, to protect itself against oxygen produced during algal photosynthesis, the cnidarian host has developed certain antioxidant defenses that are unique among animals. Finally, living in nutrient-poor waters, the animal partner has developed several mechanisms for nitrogen assimilation and conservation such as the ability to absorb inorganic nitrogen, highly unusual for a metazoan. These facts suggest a parallel evolution of symbiotic cnidarians and plants, in which the animal host has adopted characteristics usually associated with phototrophic organisms.