Seasonal changes in fecundity and fertility of the crown-of-thorns starfish, Acanthaster planci, from Davies Reef, Great Barrier Reef

Adult crown-of-thorns starfish (Acanthaster planci) were collected from Davies Reef between August 1990 and January 1992. Sampling was infrequent during the winter non-spawning period, but was as frequent as once per week during the spawning season. At each sampling, the gonad index of 10 male and 10 female starfish was calculated (gonad wet weight/total wet weight). During the winter, the total gonad weight was determined from the average weight of the gonads in three individual arms of a starfish, multiplied by the total number of arms. During the peak of spawning season (November to December), all gonads were dissected from starfish, since ripe gonads could not be assigned to a particular arm with a high degree of confidence.

On seven occasions, between October and the end of January of the 1991-92 spawning season, experiments were carried out to examine fertilization rates and development success, using starfish collected from Davies Reef. All experiments were conducted under laboratory conditions either on land or on board a research vessel. Five male and five female starfish, identified by syringe biopsy were used in each experiment. Gonad tissue (1-3 g) was dissected from each animal and placed in 50 ml of .0001M 1-methyladenine in seawater for 30 minutes. The gonads were then removed from the solution and the concentration of gametes remaining (eggs and sperm) was determined in each of the containers. Counts of sperm were obtained using haemocytometers, while small plankton sorting trays were used to count eggs in aliquots from the original suspension. Suspensions of eggs or sperm from each starfish were then adjusted to concentrations of 3x10³/ml for sperm and 3x10²/ml for eggs. Sperm from each male was crossed with eggs from each female. Controls for sperm contamination were prepared using eggs alone with no additional sperm added. For each cross eggs, or eggs and sperm, were placed into three replicate 20 ml glass scintillation vials.

The proportion of fertilised eggs was scored 2 hours after the addition of sperm. All eggs with fertilization membranes were scored as 'fertilized'. Approximately 100-150 eggs were counted. The vials containing the eggs were then resealed and re-counted 24 hours later. Eggs or embryos (100-150) were scored as either 'gastrula', if they had developed a definite archenteron, or 'non-gastrula', where this invagination was lacking or where fertilization had not occurred.