The findings, published in the Proceedings of the National Academy of Sciences (PNAS) today [22 June], offer a 500-million-year perspective on how photosymbiosis, the relationship between corals and photosynthetic algae, has shaped coral success. The new research challenges the assumption that this biological innovation has always been beneficial, showing instead that its advantage has shifted dramatically over deep time.
Rising temperatures cause bleaching, and ocean acidification attacks skeletons. But not all corals respond the same way, and the rules governing survival have changed fundamentally over Earth's history.
Of the roughly 6,000 living coral species, only half require sunlight. These colourful corals host photosynthetic algae for energy. The other half, often overlooked, thrive without algae in deeper, darker waters. Scientists classify these as Z (symbiotic) and AZ (non-symbiotic) corals.
Led by Zhengsheng Wei (China University of Geosciences, Wuhan), who began this work as a visiting student at the University of Bristol under Professor Mike Benton, with Professor Zhong‑Qiang Chen (CUG-Wuhan) and Professor Wolfgang Kiessling (FAUE-Nürnberg), the team used Bayesian modelling and AI to analyse vast fossil datasets to test how each group responded to environmental changes across geological time.
During the Paleozoic, AZ corals outpaced their Z corals. Z corals even failed to recover after the Late Devonian extinction. Both groups were similarly vulnerable to warming and anoxia.
But everything changed with the rise of scleractinian corals in the Triassic. After that, photosymbiosis became the primary driver of diversification. Z coral gained a clear advantage, not because symbiosis is inherently superior, but because the environmental context had shifted.
The study also found different dynamics: Z success was driven by origination (new species), AZ success by extinction (avoiding death).
Z corals are especially sensitive to short‑term temperature changes, likely due to their shallow habitats. High temperatures force them to expel algae, causing bleaching. Deeper‑water AZ corals can better weather such storms.
Mike Benton, Professor of Vertebrate Paleontology in the School of Earth Sciences at the University of Bristol, said: "Our work confirms the vulnerability of sunlight‑loving Z corals, but shows that deeper‑water AZ corals are less at risk. However, like all marine animals, they will eventually move away from waters that become too hot."
The study underscores that conservation strategies must account for these differences. Understanding how survival rules have changed in the past could help predict, and perhaps mitigate, future biodiversity losses in a warming world.
Paper
'The contingent advantage of photosymbiosis in coral evolution' by Zhensheng Wei, Wolfgang Kiessling, Zhen Guo, Michael J. Benton, Lewei Su, Yuangeng Huang, and Zhong-Qiang Chen in Proceedings of the National Academy of Sciences (PNAS) [open access]
