Stress, Development, and Evolution in Coral Reef Communities

Abstract

Attention to coral reef communities has increased as concerns about climate change have mounted. Integral to these reefs are colonial corals and their cnidarian relatives. Reef building depends on symbiosis with photosynthetic dinoflagellates. When environmental stress becomes extreme, these dinoflagellates are lost, and corals bleach. Symbiotic cnidarians thus return to their primitive, nonsymbiotic state. This symbiosis impacts all aspects of coral biology. Colony development begins with the formation of a primary polyp, which corresponds to development in unitary animals. The subsequent development of a colony, however, is without parallel in unitary animals and is particularly sensitive to input from environmental factors. These factors include water currents and patterns of food availability. For symbiotic cnidarians, availability of light has major impacts as well. Additionally, light functions in mechanisms of host-symbiont conflict mediation. Environmental signaling pathways are thus central to cnidarian development and result in considerable phenotypic plasticity. Plasticity poses challenges for the classification of corals. Modern molecular studies have revealed that well-known coral species are in some cases not distinct but rather either polymorphic or frequent hybridizers. At the same time, cryptic lineages—morphologically similar but genetically distinct—have been identified in other species. High levels of genetic polymorphism or reticulate evolution, or both, could have dramatic impacts on the availability of heritable variation in corals and hence their responsiveness to selection caused by climate change. These recent data suggest that much remains to be learned about the genetic and phenotypic plasticity of corals, which together may be the key to alleviating the scourge of coral bleaching. Nonetheless, evolutionary theory suggests that bleaching may be a consequence of mechanisms of conflict mediation, which are essential to maintaining the coral-dinoflagellate symbiosis.