In an experiment, warmer conditions altered egg size and sperm production in Acropora aspera, depending on how long the heat lasted.
Researchers exposed gravid Acropora aspera colonies to nine combinations of temperature and duration: the current temperature, 1°C warmer or 2°C warmer, for three, six or nine weeks. They then measured the proportion of colonies with gravid polyps, eggs per polyp, egg volume and testis volume at spawning.
The responses differed between reproductive traits. At 2°C above the control temperature for nine weeks, testis volume fell by about threefold, while a six-week exposure at the same temperature increased sperm production. Longer heat exposure increased egg volume, which nearly doubled at 1°C above the control, without changing egg counts.
How heat changed reproduction
Heat stress altered coral reproductive investment in ways that depended on both intensity and duration. Sperm production was the most sensitive measure: testis volume declined by about threefold after nine weeks at 2°C above the control temperature, but six weeks at the same temperature increased sperm production.
Egg responses followed a different pattern. Longer exposure to heat stress increased egg volume, with egg volume nearly doubling at 1°C above the control temperature, while the number of eggs per polyp remained stable. The researchers say this may indicate investment in fewer, better-provisioned eggs rather than more eggs, but they also note that larger eggs may reflect stress-related physiological changes rather than improved egg quality.
Why timing matters
The findings show that heat stress before spawning does not produce one uniform reproductive response. Its effects depend on how warm conditions are, how long they last and when they occur during gamete development.
On the Great Barrier Reef, heat events currently overlap with early egg development in some abundant corals, and earlier heat events are expected to increasingly overlap with later stages as the climate changes. Accounting for these different responses could improve predictions of future coral reproduction and reef recovery potential.
Evidence and caveats
This was an ex-situ experiment on gravid Acropora aspera colonies exposed to controlled combinations of temperature and duration. It measured reproductive traits at spawning, including egg and testis volume, egg counts and the probability of gravid polyps.
The abstract does not give the number of colonies or report whether the treatments affected the quality or survival of eggs, sperm or larvae. In addition, the experiment studied one coral species under laboratory conditions, so the results may not apply to all corals or to natural reef environments. Larger egg volume also did not necessarily mean better egg quality.
