Mass spawning in the Acroporidae on the Ningaloo Reef Tract

The development and maturation of oocytes, and the timing of gamete release was documented for four Acroporidae, Acropora formosa, Acropora hyacinthus, Montipora capricomis and Montipora spumosa. Maturation was determined by the pigmentation seen in oocytes altering their appearance from white to pink or red in the Acropora spp. and from white to brown in the Montipora spp. The maturation of oocytes increased rapidly in the two weeks before gamete release. This study has shown that sexual reproduction in the broadcast spawning Acroporidae occurred over the March to May period on the Ningaloo Reef Tract (NRT) in 1993. Interspecific synchrony in coral spawning was recorded within the region of Coral Bay and spawning was predominantly confined to a period extending from the 8th to 10th nights following the full moon in April. Over this period, synchronization of gamete release was recorded in 61.5% of the population of four Acroporidae, and observed in situ for many other species. Whilst this major period of gamete release was observed for Acropora formosa, A.hyacinthus, Monitpora capricomis and Montipora spumosa, a proportion of the population for each species did not participate in the major April Full Moon (FM) spawning event thus gametogenesis is approximately synchronous. Spawning extended over three consecutive months during late summer/autumn, at times associated with the Full and New moons between March and May. Spawning periods were observed between the 6th and 11th nights following either a New or Full moon. Only a small proportion (less than 10%) of each species spawned in periodocity with the New moons between March and May, however a significant proportion of the population participated in spawning events following the Full moons over this time. Furthermore a significant proportion (22%) of the two Montipora species, released gametes over the May Full moon spawning period. The way in which a proportion of the population co-ordinates the timing of gamete release to occur over brief distinct periods leads to seasonality in reproduction and ultimately a major spawning event, of which participating species can be referred to as 'quasi-mass spawners'. Although similarities exist between the synchronosity in spawning seen on the Ningaloo Reef Tract and the Great Barrier Reef, spawning on the Ningaloo Reef Tract occurs 4-5 months after mass spawning on the Great Barrier Reef and results from the present study show that there is no evidence for the occurance of a spring spawning event at Ningaloo Reef. The total mean oocyte volume (mm3) per polyp was recorded for the four species and ranged from 0.18mm3 to 0.68mm3 Montipora spp. were shown to exert a lower reproductive effort than Acropora spp. which typically had large but fewer oocytes within the polyp, than Montipora spp. Acropora hyacinthus had an average of 7.0 mature oocytes per polyp and A.formosa had 9.8 oocytes per polyp which was less than the two Monitpora species, M.capricomis and M.spumosa with 13.0 and 11.0 oocytes per polyp respectively. Mean oocyte volume varied significantly between all species except A.hyacinthus and A.formosa, where the difference between mean oocyte volume of these species did not vary to a significant level. This shows that these two species of common genera (Acropora ) were not shown to vary significantly in either oocyte volume or mean number of oocytes per polyp. The total mean oocyte volume is a measure of reproductive effort, and presents here, data which has not previously been recorded for these species on the NRT. For the purpose of studies which may document the reproductive effort of these species at a later date, the results presented here can provide determining potential as indices of sub-lethal stress