The biology of coral metamorphosis: molecular responses of larvae to inducers of settlement and metamorphosis

Like many other cnidarians, corals undergo metamorphosis from a motile planula larva to a sedentary polyp. In some sea anemones such as Nematostella this process is a smooth transition requiring no extrinsic stimuli, but in many corals it is more complex and is cue-driven. To better understand the molecular events underlying coral metamorphosis, competent larvae were treated with either a natural inducer of settlement (crustose coralline algae chips/extract) or LWamide, which bypasses the settlement phase and drives larvae directly into metamorphosis. Microarrays featuring > 8000 Acropora unigenes were used to follow gene expression changes during the 12 h period after these treatments, and the expression patterns of specific genes, selected on the basis of the array experiments, were investigated by in situ hybridization. Three patterns of expression were common—an aboral pattern restricted to the searching/settlement phase, a second phase of aboral expression corresponding to the beginning of the development of the calicoblastic ectoderm and continuing after metamorphosis, and a later orally-restricted pattern.