As ocean temperatures rise, corals' ability to withstand and adapt to heat stress is critical for the future of coral reefs. Many reef-building corals already live near their upper thermal limits, making them vulnerable to bleaching and, with prolonged stress, mortality.
But corals don't all respond to heat the same way. Thermal tolerance can vary among species, populations, and individual colonies, and may be shaped by genetics and the environments in which they live. Understanding these differences could help to predict which corals are more resilient as oceans warm.
Hawaiʻi offers a unique setting to study this question. Nearby reefs in Kāneʻohe Bay and Kailua Bay experience markedly different conditions, with Kāneʻohe warmer and more sheltered and Kailua cooler and more exposed. This contrast enables researchers to examine whether a coral's environmental history influences its ability to withstand heat stress.
A Florida Atlantic University Harbor Branch Oceanographic Institute study, in collaboration with the University of Hawaiʻi, compared the thermal tolerance of two abundant reef-building corals: Montipora capitata, or rice coral, which can form broad plates or branching colonies with a textured surface; and Porites compressa, or finger coral, which grows in clusters of slender, finger-like branches.
Coral fragments from both species were collected in Kāneʻohe Bay and Kailua Bay and exposed to controlled thermal stress in the laboratory. Researchers used Pulse Amplitude Modulation fluorometry to measure photosynthetic efficiency as an indicator of physiological response to warming. They then compared corals from the two environments to determine whether thermal tolerance varies by origin and whether the pattern is consistent across species.
Results of the study, published in Scientific Reports , reveal that the ability of corals to withstand heat stress can vary not only by species, but also by where they live. Both species showed substantial thermal tolerance, with bleaching thresholds above the 4-degree heating week level that NOAA uses as an indicator of accumulated heat stress associated with the onset of significant coral bleaching.
However, one species stood out: P. compressa from the cooler, more exposed waters of Ulupaʻu had significantly higher thermal tolerance than P. compressa from warmer, more sheltered Kāneʻohe Bay and, than M. capitata from either location.
For M. capitata, thermal tolerance was remarkably similar between the two bays. At Ulupaʻu, however, P. compressa tolerated significantly more accumulated heat stress than M. capitata. The researchers also found that P. compressa bleached more slowly than M. capitata, suggesting that different coral species may rely on distinct physiological strategies to cope with warming.
"Our findings highlight that coral resilience to ocean warming is not uniform, even among species living in close geographic proximity," said Joshua D. Voss, Ph.D., senior author and research professor at FAU Harbor Branch. "As reefs around the world face more frequent and intense marine heat events, understanding which corals can tolerate that stress – and why – will be critical for protecting and restoring reefs. What works for one species or population may not work for another, so conservation strategies need to account for both the biological and the environmental diversity that exists within coral reef ecosystems."
The researchers found that the differences were not simply a matter of one bay producing universally tougher corals. Within Kāneʻohe Bay, the two species had similar thermal thresholds, while at Ulupaʻu, P. compressa had a clear advantage. The finding suggests that environmental conditions may shape thermal performance differently across species. Ulupaʻu's cooler, more variable and wave-exposed environment may provide repeated exposure to changing conditions that helps prepare some corals for heat stress, although the study did not directly test the underlying mechanisms.
"By exposing corals from distinct environmental histories to a standardized thermal stress experiment, we were able to directly compare differences in thermal tolerance across species and populations," said Allison M. Klein, Ph.D., first author and a postdoctoral fellow at FAU Harbor Branch. "This approach allowed us to examine how prior environmental conditions may shape physiological responses to heat stress."
The findings also have implications for coral restoration. P. compressa from Ulupaʻu may represent one source of naturally heat-tolerant corals, but the researchers caution against assuming that the most heat-tolerant corals are always the best candidates for restoration. Maintaining genetic diversity and a range of traits could be important for giving restored reefs the flexibility to respond to different environmental conditions over time.
"This study identified more resilient corals from certain locations in these testing conditions, which provides important hope for the future," said Voss. "The results demonstrate that both species identity and local environmental history can influence coral thermal performance and success in coral restoration efforts."
This research was conducted in collaboration with the Coral Resilience Lab at the Hawaiʻi Institute of Marine Biology, under the advisement of Crawford Drury, Ph.D. Coral collection and thermal performance assays were conducted at the lab.
The project was supported by the Defense Advanced Research Projects Agency, the U.S. Geological Survey, and an award from the Robertson Family Foundation to FAU Harbor Branch. Additional support was provided through U.S. Geological Survey Cooperative Agreement No. G21AC10662-01.
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