Spermcasting is an unusual reproductive strategy in some aquatic species in which functional males release gametes to fertilize eggs inside the body cavity of functional females. As some spermcasting bivalves have high ecological and economical importance, the understanding of their reproductive strategy is crucial for aquaculture, fisheries and conservation. This study investigates gametogenesis, sex ratio and energy metabolism in Ostrea angasi to elucidate the reproductive strategy of spermcasting bivalves. Gonad histology indicated asynchronous development of spermatozeugmata, but synchronous development of oocytes within an individual. The hermaphroditic individuals released spermatozeugmata before egg ovulation. The population of 2-3 year old oysters comprised 46.7% hermaphrodites and showed a highly skewed male to female ratio of 7:1. This species primarily metabolized glycogen as the energy source for gametogenesis, with an overlapping period of energy storage and utilization. This pattern of gametogenesis suggests multiple production of spermatozeugmata in a reproductive season in male and hermaphroditic oysters, but a single episode of egg ovulation in female and hermaphroditic oysters. The dynamics of energy metabolism indicate that O. angasi follows a strategy for energy metabolism that is intermediate between those of conservative and opportunistic species. This study suggests that unsynchronized gamete development, skewed sex ratio and intermediate energy metabolism are adaptive strategies in the reproduction of spermcasting bivalve species.