Abstract
Repeated bleaching events driven by heat stress have caused extensive coral mortality and declines in survivors’ reproductive output, undermining reef recovery. On the Great Barrier Reef, heat stress events currently coincide with early oogenesis in many abundant taxa; with climate change, earlier onsets are expected to increasingly overlap later gametogenic stages. How coral reproduction responds as gametes mature in warmer waters remains unclear. To address this, we quantified the effects of heat stress during late gametogenesis on the fecundity of Acropora aspera . Using an ex situ experiment, gravid colonies were exposed to nine treatments combining three temperatures (Control + 1, + 2 °C) and three durations (3, 6, 9 weeks). Gravid polyp probability, polyp egg counts, egg volume, and testis volume were assessed at spawning. Responses were trait- and duration-specific. Sperm production was most sensitive, with testis volume declining ~ threefold under + 2 °C for 9 weeks relative to controls (0.056 mm 3 vs 0.160 mm 3 ); the same exposure nearly halved gravid polyp probability (0.015 vs 0.026 in Controls). However, 6-week exposure at + 2 °C increased sperm production (0.174 mm 3 ), highlighting the influence of exposure duration. In contrast, longer exposure to heat stress increased egg volume, which nearly doubled at + 1 °C (0.176 mm 3 vs 0.098 mm 3 in Controls); while egg counts remained stable, suggesting a strategy favouring better provisioned rather than numerous eggs. Nevertheless, larger gamete volumes may reflect stress-induced physiological changes rather than improved quality. These findings demonstrate that heat intensity, duration, and timing jointly shape coral reproductive investment, highlighting the complexity of predicting future reef recovery under climate change.