The experiment was designed around a deceptively simple question: does a coral’s environmental history shape how it responds to heat stress? Divers collected lobes from eight healthy O. annularis colonies at each of four reef sites in January 2023, cutting them into 128 fragments of roughly 2.5 by 2.5 centimeters. Two of the sites, Rupert Rock and Hassel Island, sit close inshore and are more influenced by land-derived runoff; the other two, Flat Cay and Saba Island, lie farther offshore in clearer water. After a month of healing on terracotta tiles, the fragments were reciprocally transplanted so that every source population was represented at every site, with 32 fragments per location.

The timing could hardly have been more consequential. Water temperatures began rising in April 2023 and crossed the regional bleaching threshold of 29.43 degrees Celsius by late June, remaining above it through November. Cumulative heat stress, measured in Degree Heating Weeks, exceeded 20 DHW at all four sites — a level NOAA associates with its highest alert category and a risk of near-complete coral mortality. Peak values reached 24.24 DHW at Rupert Rock and 23.84 DHW at Hassel Island, with the offshore sites close behind at roughly 21 DHW.

Against that backdrop, the researchers tracked fragment growth, bleaching, and mortality monthly, photographing each fragment and analyzing tissue area with image software. Growth told a nuanced story. Relative growth increased significantly through time across all treatments, meaning the corals kept adding tissue even as the heatwave intensified. But growth trajectories diverged depending on both where a fragment came from and where it was placed, and there was no consistent evidence of a home-site advantage. In fact, fragments transplanted away from their natal reef grew significantly faster over time than those left at home — a finding that runs counter to some earlier reciprocal transplant studies.

The most striking growth result involved Hassel Island, the shallowest site at 1.8 meters and the one with the highest measured light levels. Fragments moved from Rupert Rock to Hassel Island posted the highest mean relative growth of any treatment, and fragments transplanted to Hassel Island generally outgrew those sent to Rupert Rock regardless of origin. Elevated irradiance can fuel photosynthesis and carbon transfer to the coral host, but the authors caution that under prolonged thermal stress, high light can also carry costs such as photoinhibition and calcification trade-offs that a two-dimensional tissue metric would not capture.

Bleaching, unsurprisingly, became widespread across every combination of source and transplant location as heat accumulated. Neither origin nor destination had a significant overall effect on the probability that a fragment would bleach. Yet one signal cut through the noise: fragments originating from Hassel Island crossed the 10 percent bleaching threshold significantly later than fragments from Flat Cay and Saba Island, with hazard ratios of 2.80 and 2.56 respectively. The delay followed the fragments wherever they were transplanted, pointing to something carried from their home reef rather than conditions at the transplant site.

Crucially, that head start did not translate into a better outcome. An analysis of maximum bleaching severity found no differences among source or transplant locations — Hassel Island corals eventually bleached just as extensively as everyone else. The researchers interpret this as evidence that environmental history may shape the timing of a coral’s thermal response more than its ultimate severity. Hassel Island’s exceptionally high light availability is one candidate explanation, since corals from bright, warm environments have sometimes shown enhanced thermal tolerance, but because the site is also much shallower than the others, the effects of light and depth cannot be disentangled in this study.

Mortality told a more localized story. Complete death of a fragment was relatively uncommon — just 9 of 128 fragments — but 8 of those events occurred at Rupert Rock, and 7 of those were concentrated on a single array. Before dying, those fragments showed rapid tissue loss with distinct, even lesion edges, a pattern consistent with disease progression. The array sat adjacent to a sand patch, and sediments have been implicated as vectors for stony coral tissue loss disease. When the researchers excluded that single array in a sensitivity analysis, survivorship differences among transplant locations vanished entirely, a caution against reading site-wide effects into what may have been a localized outbreak.

For restoration practitioners, the findings carry practical weight. Coral gardening programs increasingly seek to identify heat-tolerant source colonies, and the delayed bleaching of Hassel Island-sourced fragments suggests that source populations can differ in when — not just whether — they respond to acute stress. But the authors are careful to note that this does not make Hassel Island a universally superior source of stock. The pattern was observed during a single extraordinary event, in one species, over one year, and the eventual bleaching severity was identical across origins.

The broader lesson may be about the limits of prediction under extreme stress. The 2023 heatwave was severe enough that it appears to have overwhelmed many differences in final bleaching outcomes, even as differences in onset timing remained detectable. As marine heatwaves grow more frequent and intense, the study suggests that identifying resilient corals will require looking across multiple measures of performance — growth, bleaching timing, bleaching severity, and survival — and across multiple events, because a coral’s history and its transplant destination each leave their mark on different aspects of the response.

Subject of Research: Reciprocal transplant experiment on Orbicella annularis coral responses to the 2023 mass bleaching event in St. Thomas, US Virgin Islands

Article Title: Bleaching and mortality responses of Orbicella annularis during the 2023 mass bleaching event in St. Thomas, USVI

Article References: Shelby, A., Meiling, S. S., Heidmann, S., Brandt, M. E., & Smith, T. B. (2026). Bleaching and mortality responses of Orbicella annularis during the 2023 mass bleaching event in St. Thomas, USVI. Frontiers in Marine Science, 13, Article 1895022. https://doi.org/10.3389/fmars.2026.1895022

Image Credits: AI Generated

DOI: 10.3389/fmars.2026.1895022

Keywords: coral bleaching, Orbicella annularis, coral restoration, marine heatwave, reciprocal transplant, thermal tolerance, U.S. Virgin Islands, degree heating weeks, environmental history, coral mortality, reef conservation, stony coral tissue loss disease