Reproductive seasonality and biannual spawning of Acropora on two north-west Australian reefs

Studies of coral spawning are necessary for the adequate management of coral reef ecosystems due to the environmentally sensitive nature of the coral spawning period, when millions of coral eggs are released and float to the surface of the ocean. Biannual spawning refers to the incidence of broadcast spawning corals that spawn twice annually, instead of once that is typical of most broadcast spawning species in Australia. Two species of Acropora that had been observed by the Australian Institute of Marine Science to have mature gametes in both October 2002 and February 2003 in the Dampier Archipelago, Acropora samoensis and Acropora cytherea, were examined to determine whether individual colonies had mature gametes twice, in both March and October, or whether individual colonies within the population were spawning at two different times. This study showed that each individual colony produced mature gametes only once, in either October or February. In A. samoensis 55 % of colonies had ripe gametes in October, the other 45 % had ripe gametes in February (n= 11) while in A. cytherea 80 % of colonies had ripe gametes in October, and 20 % in February (n=5). The number of eggs per polyp was measured in both October and February in A. samoensis, with averages of 10.56 eggs per polyp and 10.24 respectively, indicating that there was no difference in reproductive output between the October and March spawners. Seasonality of spawning was investigated further south at Ningaloo Reef (Coral Bay region) to determine if spawning occurred in October there also. Thirty-one species across five families were examined in October and there was no sign of mature gametes in any. The two species with mature gametes in October that were found in Dampier could not be located in Coral Bay, so it is still not known whether they are spawning twice at Ningaloo Reef also. Egg development was monitored from October to March in six sympatric species of Acropora at Coral Bay. Five species were found to have mature gametes in March, and participated in the mass spawning event 7-8 days after the full moon in March. Mass spawning was monitored and the proportion of colonies that participated in the event ranged from 83-100 % among the five species. Coral spawning occurred in the last quarter lunar phase, after dark, on an ebbing tide as tides were approaching neap. One species, Acropora papillare, did not spawn in the mass spawning event, but was found with mature gametes in October and January. Spawning is assumed to have taken place in December and January in this species. Fecundity and oocyte size were measured in October, January and March in all six species and oocyte degeneration was observed between January and March in three species. Reproductive output (number of eggs per polyp and oocyte diameter) recorded in this study was compared with the figures in the World-wide Acropora database, which showed that reproductive output was greater at Ningaloo than elsewhere, in all four species in which comparisons could be made