The first scleractinians, progenitors of modern corals, began to appear 240 million years ago; by the late Jurassic (150 Ma) most families of modern corals had evolved and begun forming reefs (1, 2). Mechanisms controlling the recruitment of new corals to sustain these structures are, however, poorly understood (3). Corals, like many marine invertebrates, begin life as soft-bodied larvae that are dispersed in the plankton (3, 4). As the first step in developing a calcified coral colony, the larva must settle out of the plankton onto a suitable substratum and metamorphose to the single calcified polyp stage cemented to the reef (3, 5). Our analyses of the metamorphic requirements of larvae in divergent coral families surprised us by revealing the existence of a common chemosensory mechanism that is required to bring larvae out of the plankton and onto the reef. This mechanism appears to be quite old, predating both the phylogenetic divergence of these coral families and the development of different modes of coral reproduction.

Reef-building corals, which reproduce through simultaneous multispecies spawning, are thought to hybridize frequently, and it is hypothesized that they have evolved in repeated rounds of species separation and fusion. We conducted cross-fertilization experiments and molecular analyses with a number of mass-spawning coral species in the genus Acropora. A high rate of interspecific fertilization occurred between some species despite very different morphologies. The hybrid larvae developed normally and contained an allelic sequence transmitted from each parent, suggesting common diploid hybridization. Molecular phylogenetic analyses provided strong evidence for a gene pool shared between the hybridizing species. These reproductive and genetic characteristics are consistent with a species complex formed under the separation/fusion processes predicted for a reticulate evolutionary history.

Field observations of spawning behavior of scleractinian corals around Akajima Island were carried out from late spring to summer in 1989, 1990 and 1991. Investigations focused on the degree of spawning synchrony among Acropora spp. and its relation to fluctuations in environmental factors. Eighty-five species, representing 27 genera and 10 families of scleractinian corals, were observed to spawn from May to early September during the 3 years. Spawning of most Acropora spp. took place synchronously but timing varied between the 3rd night before to the 7th night after full moon in May and/or June. Non-Acropora species spawned mainly from the 2nd to the 8th night after full moon from June to August. The relationship between date of spawning and lunar phase was not clear, but other environmental stimul~ such as marked changes of temperature, salinity and current velocity might trigger mass synchronous spawning among the Acropora.