Coral larvae: From gametes to recruits

The life-cycle of reef-building corals, as is true for many benthic marine invertebrates, includes a planktonic larval phase that is critical to the maintenance of adult populations. Interest in understanding the biology of this larval phase has taken on a heightened sense of urgency as the unprecedented decline of reef-building corals throughout the world raises fears that the supply of sexually produced larvae may no longer be sufficient to sustain reef ecosystems. Here we provide an overview of coral larval biology by integrating advances that have been made using both physiological and ecological approaches. The primary areas addressed include 1) a brief survey of reproductive modes as well as aspects of development, including the onset of competence, 2) an analysis of the abiotic and biotic factors that affect planula larvae during dispersal and an assessment of the spatial scales over which these various parameters act, and 3) an accounting of the exogenous synthetic and tentative endogenous factors that induce larva-to-polyp transformations in vitro as well as efforts to decipher the underlying signaling pathways. We conclude that the relatively recent discovery of mass spawning events exhibited by many of the major reef-building corals throughout the Pacific and Caribbean has done much to advance our understanding of coral larval biology because it has allowed researchers to obtain gametes that can be used in a wide range of physiological and ecological studies. We also recommend the following as high priority research areas: 1) the mechanisms involved in developing metamorphic competence, 2) the synergistic effects of biotic and abiotic factors on the dispersal and settlement of coral larvae, and 3) comparative studies of the putative endogenous morphogens that orchestrate larva-to-polyp metamorphosis.