Seasonal reproduction in equatorial reef corals

Populations of broadcasting reef corals often exhibit marked reproductive seasonality and spawning synchrony. Within speciose coral assemblages there is often considerable overlap of spawning periods among species, resulting in multi-species spawning events (or "mass coral spawning"). Earlier geographical comparisons of reproductive synchrony suggested a reduction in the extent of mass spawning with proxiniity to the equator. In contrast, recent studies have revealed that reproductive seasonality and spawning synchrony within and among species are features of coral assemblages on equatorial reefs. Here we review the proposed causes of synchronous spawning among reef corals and discuss how recent findings about reproduction of corals from Singapore's equatorial reefs shed light on these various theories. Sexual reproduction in broadcasting corals requires external fertilization, so reproductive seasonality (leading to spawning synchrony) within populations is probably highly adaptive because synchrony increases the chances of gametes meeting, enhances the possibility of outbreeding and may swamp opportunistic predators. No coastal location is truly aseasonal, with even equatorial reefs experiencing marked (albeit less pronounced) rhythmic changes in sea surface temperature. Consequently, if species respond similarly but independently to timing cues to synchronize reproduction within populations, mass spawning is just as likely to occur in equatorial coral assemblages as it is at higher latitudes.