The mass spawning of corals, at an "inshore" reef in the central Great Barrier Reef Province, has consistently occurred during the week following the October full moon, in the past 4 years. One annual mass spawning period was observed when the full moon fell in the second half of October. In contrast, the mass spawning period was split, in 2 years when the full moon fell in the first half of October (two spawning periods separated by 1 lunar month). Three years of data, at an "offshore" reef at approximately the same latitude, confirm that the offshore mass spawning period occurs exactly one lunar month later than the "inshorel" spawning period.
In 1984, spawning during a mass spawning period was documented for 25 scleractinian species which had not been studied during previous spawning periods. The cumulative number of mass spawning species on the Great Barrier Reef now totals 133 (total nLlllber of species = 356). In comparison, other studies have found 14 species on the Great Barrier Reef which planulate or spawn only at other times of the year. The mode of release of reproductive products has been documented for 12 of the 15 Great Barrier Reef families. The Faviidae, Oculinidae, Mussidae, and Pectiniidae so far contain only mass spawning species. The Acroporidae is by far the largest family, and of species studied to date, 88% spawn during a mass spawning period. It is therefore likely that the majority of corals on the Great Barrier Reef participate in the annual mass spawning phenomenon.
A preliminary analysis of reproductive rhythms of mass spawning corals is presented. Lunar and diel light cycles have been identified as zeitgebers for spawning synchrony. Variation in the number of synodic months between annual mass spawning periods indicates that an endogenous rhythm, if present, must interact with the lunar cycle. Such an interaction would maintain annual mass spawning periods within relatively constrained environmental limits, and result in reasonably precise seasonality of spawning.