Juveniles exhibit lower heatwave tolerance than adults in a reef-building coral
- 1Palau International Coral Reef Center
Abstract
Marine heatwaves cause widespread coral bleaching and mortality, yet thermal responses often differ between adult and juvenile corals, with the latter frequently being more tolerant. Whether the observed enhanced tolerance of juveniles reflects intrinsic physiological differences across life stages or simply reduced environmental stress due to shaded microhabitats occupied by juveniles, or a combination of both factors, remains unclear. To test whether colony size determines thermal responses and if a threshold size exists for tolerance, we exposed 89 colonies of the reef-building coral Acropora aff. digitifera to a 6-week, 32.5 C simulated marine heatwave under controlled light conditions, with cumulative stress reaching 20 C-weeks. Contrary to expectations, ~2 C-weeks less heat stress was required to trigger bleaching and mortality in coral juveniles compared to adults. Mortality divergence across colony sizes was greatest at ~10 C-weeks, when the smallest corals exceeded 80% mortality, whereas the largest colonies showed <10% mortality. A breakpoint in survival odds was apparent at 13.4 cm diameter, corresponding to the size at onset of sexual maturity in the native wild population for this species. These results indicate an ontogenetic shift in physiological heat tolerance, with larger reproductively mature colonies showing enhanced tolerance. The apparent survival advantage of juveniles in the reef frequently reported during bleaching events could be more likely due to micro-environmental buffering than intrinsic physiological tolerance. Our findings suggest that juvenile corals may be more vulnerable to climate change than previously thought, particularly as declining habitat complexity could reduce microhabitat refugia on future reefs.
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BibTeX
@misc{alessi2026juveniles,
title = {{Juveniles exhibit lower heatwave tolerance than adults in a reef-building coral}},
author = {C. Alessi and L. Lachs and A. Humanes and H. Martinez and N. Croft and L. Manuel and P. J. Mumby and A. I. Tellei and D. Cassidy and J. Guest},
howpublished = {bioRxiv},
year = {2026},
month = sep,
doi = {10.64898/2026.09.23.753667},
url = {https://www.biorxiv.org/content/10.64898/2026.09.23.753667v1},
note = {CC BY-ND},
}
RIS
TY - UNPB
TI - Juveniles exhibit lower heatwave tolerance than adults in a reef-building coral
AU - C. Alessi
AU - L. Lachs
AU - A. Humanes
AU - H. Martinez
AU - N. Croft
AU - L. Manuel
AU - P. J. Mumby
AU - A. I. Tellei
AU - D. Cassidy
AU - J. Guest
PY - 2026
DA - 2026/09/23
PB - bioRxiv
DO - 10.64898/2026.09.23.753667
UR - https://www.biorxiv.org/content/10.64898/2026.09.23.753667v1
AB - Marine heatwaves cause widespread coral bleaching and mortality, yet thermal responses often differ between adult and juvenile corals, with the latter frequently being more tolerant. Whether the observed enhanced tolerance of juveniles reflects intrinsic physiological differences across life stages or simply reduced environmental stress due to shaded microhabitats occupied by juveniles, or a combination of both factors, remains unclear. To test whether colony size determines thermal responses and if a threshold size exists for tolerance, we exposed 89 colonies of the reef-building coral Acropora aff. digitifera to a 6-week, 32.5 C simulated marine heatwave under controlled light conditions, with cumulative stress reaching 20 C-weeks. Contrary to expectations, ~2 C-weeks less heat stress was required to trigger bleaching and mortality in coral juveniles compared to adults. Mortality divergence across colony sizes was greatest at ~10 C-weeks, when the smallest corals exceeded 80% mortality, whereas the largest colonies showed <10% mortality. A breakpoint in survival odds was apparent at 13.4 cm diameter, corresponding to the size at onset of sexual maturity in the native wild population for this species. These results indicate an ontogenetic shift in physiological heat tolerance, with larger reproductively mature colonies showing enhanced tolerance. The apparent survival advantage of juveniles in the reef frequently reported during bleaching events could be more likely due to micro-environmental buffering than intrinsic physiological tolerance. Our findings suggest that juvenile corals may be more vulnerable to climate change than previously thought, particularly as declining habitat complexity could reduce microhabitat refugia on future reefs.
ER -