MONSOONALLY DRIVEN REPRODUCTION IN THE TROPICAL BLACK-LIP ROCK OYSTER SACCOSTREA ECHINATA (QUOY & GAIMARD, 1835) IN NORTHERN AUSTRALIA

The reproductive cycle of the tropical black-lip rock oyster Saccostrea echinata was determined at three locations in the Northern Territory, Australia. Wild oysters were collected every 4-6 wk (n = 13-23) from January 2016 until June 2017. Biometric and condition index data were collected, and tissues were histologically examined to determine gonad index (GI) and gender ratios. Seawater and air temperature were monitored by intertidally deployed data loggers, and daily rainfall data were obtained from local weather stations. A total of 527 oysters were sampled across all sites, of which 34.2% were females, 47.4% were males, 0.4% were hermaphrodites, and 18.0% were of indeterminate gender. The GI presented a clear annual cycle; values fluctuated but remained high (1.94-3.94) from October through April and were low (>1.50) from May through September, across both years. Across all sites, the GI had a strong, positive correlation with temperature (r = 0.783; df = 26; P <0.001) and a moderate, positive correlation with rainfall (r = 0.496; df = 31; P <0.05). Synchronization in spawning patterns occurred between sites, and gender ratios suggest that S. echinata is a dioecious species with low levels of functional hermaphroditism. The major finding of this study is that S. echinata spawns semicontinuously throughout the monsoon season (October-April) and has an extended resting phase throughout the dry season (May-September). The information generated in this study has a number of potential applications relating to the timing of broodstock collection and hatchery production, and it provides a broader basis for further developing S. echinata as a commercial aquaculture species.