Effects of pre-spawning heat stress on the fecundity of Acropora aspera

<title>Abstract</title> Repeated bleaching events driven by heat stress have caused extensive coral mortality and long-term impairment in survivors. Following bleaching, larval production and recruitment decline, undermining reef recovery. On the Great Barrier Reef, such heat-stress events currently coincide with early oogenesis in abundant taxa; with climate change, earlier onsets are expected to overlap increasingly with later gametogenic stages. How coral reproduction will be altered as gametes mature and spawn in warmer waters remains unclear. To address this, we quantified the effects of heat stress during late gametogenesis on the fecundity of <italic>Acropora aspera</italic> . Using an <italic>ex-situ</italic> experiment, gravid colonies were exposed to nine treatments combining three temperatures (+ 0, + 1, +2°C) and three durations (3, 6, 9 weeks). Gravid polyp probability, egg count per polyp, 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. However, 6-week exposure at + 2°C increased sperm production, highlighting the influence of exposure duration. In contrast, longer exposure to heat-stress increased egg volume, which nearly doubled at + 1°C; while egg counts remained stable, suggesting a strategy favouring better provisioned rather than numerous eggs. Nevertheless, larger gamete volumes may not necessarily represent improved quality and could instead reflect stress-induced physiological changes. These findings demonstrate that heat intensity, duration, and timing jointly shape coral reproductive investment. Incorporating these dynamics in models is important for predicting future reproductive potential and reef resilience under climate change.