Widespread coral bleaching across subtropical and temperate Japan under record marine heat stress

The record-breaking marine heatwave of 2024 provided a natural experiment for evaluating the vulnerability of coral communities across a broad latitudinal gradient. Our surveys revealed that extreme thermal stress caused widespread bleaching and mortality from subtropical reefs to the northernmost coral communities in Japan, affecting tropical, temperate-endemic, and poleward-expanding species. Furthermore, our results demonstrated that susceptibility to thermal stress varied substantially among species and locations, highlighting the importance of community composition, local environmental conditions, and population-specific responses in shaping bleaching outcomes. The unprecedented spatial extent and severity of bleaching documented in this study underscore the increasing exposure of high-latitude coral communities to climate extremes and provide evidence for reassessing current assumptions regarding the resilience and refuge potential of temperate regions31,32,33 under ongoing climate change.

A long-term coral monitoring program has been conducted by the Environmental Agency and the Ministry of the Environment, Japan, since 2000, and coral bleaching associated with thermal anomalies has been repeatedly documented at subtropical reef sites in the Ryukyu Islands35,36. At Shiraho Reef, Ishigaki Island, extensive coral bleaching and subsequent declines in coral cover occurred during thermal anomalies in 1998 and 200735. Coral cover declines were also observed during the 2007 and 2016 bleaching events across Ishigaki Island36. Another severe bleaching event occurred in 2022, during which approximately 63% of corals bleached and 15% died at Sekisei Lagoon, Ishigaki Island37. In 2024, although the DHW reached its highest level (9.23 °C-weeks) since 1982 and widespread bleaching was observed, post-bleaching surveys around Ishigaki Island reported little reduction in coral cover38, indicating low mortality and substantial recovery. This may reflect the prior loss of thermally sensitive corals in 2022. Indeed, thermally sensitive species such as Seriatopora and Acropora exhibited higher levels of bleaching and mortality than Galaxea and Montipora species during the 2022 event37, suggesting a potential shift in community structure toward stress-resistant species following repeated bleaching events. In addition, two typhoons passed near Ishigaki in July and August 2024, whereas only one typhoon occurred in September 2022, likely reducing thermal stress and light intensity and thereby limiting mortality.

Similar to Ishigaki Island, coral communities on Okinawa Island have experienced repeated thermal stress since the 1980s39,40,41,42,43,44. In 1980, more than 40% of corals bleached and 10% died at Sesoko39. The 1998 mass bleaching event affected coral communities across Okinawa Island, and coral cover declined by 85% at Sesoko40. Substantial declines in branching Acropora, Pocillopora and Porites species at Sesoko in 1998 demonstrated that bleaching events can drastically alter coral community structure, resulting in shifts between “winner” and “loser” corals40. Additional bleaching events occurred in 2001, 2003, and 200741, and coral cover in shallow reefs around Okinawa Island declined from 24.4% to 7.5% between 1995 and 200942. Further thermal stress and bleaching events occurred in 2017 and 2022 at Okinawa Island, including the Sesoko site, with mean bleaching prevalence of 33.2% and 9.4%, respectively44. However, coral cover increased from 13.8% to 28.7% between 2017 and 2023, suggesting limited mortality and partial recovery despite bleaching44. This coral recovery was primarily driven by fast-growing Acropora and Montipora species44. In contrast, the extreme thermal stress in 2024 caused a drastic decline in coral cover, from 20 to 6%, on shallow reefs at Sesoko, largely reflecting the loss of Acropora species. Similar dynamics in coral cover associated with repeated bleaching events, in which Acropora species contributed disproportionately to both recovery and decline, have also been documented on the Great Barrier Reef45. These patterns suggest that frequent and repeated bleaching events are cumulatively reshaping coral reef systems, driving not only short-term fluctuations in coral cover but also longer-term shifts in community composition. The pace of these changes may further accelerate under ongoing climate change, potentially reducing the stability and recovery capacity of these reef ecosystems.

In contrast to subtropical coral communities, relatively few studies have documented bleaching caused by thermal stress in temperate regions around the main Island of Japan. In 1998, coral bleaching was first recorded at temperate sites such as Kochi and Kushimoto when DHW exceeded 6 °C-weeks20. Subsequent events occurred in 2010 (cold stress) and 2016 (heat stress), including at Tatsukushi and Kushimoto, however, significant mortality was not observed, and major disturbances in temperate coral communities were primarily attributed to crown-of-thorns starfish predation and typhoons43. In contrast, the 2024 marine heatwave caused extensive bleaching and mortality of corals, including at the northernmost sites that, to our knowledge, had never previously experienced thermal-stress bleaching, such as Tsushima (34° 11’ N). Additionally, at other high-latitude sites, including Tagojima (34° 48’ N) and Numazu (35° 02’ N), no bleaching had been recorded prior to 2023; however, bleaching occurred consecutively in 2023 and 2024. In 2023, pale and bleached corals were first observed by local divers, coinciding with DHW values of 12.3 and 10.8 °C-weeks, respectively (Fig. 4). In 2024, these high-latitude sites experienced even higher DHW values exceeding 17 °C-weeks, resulting in extreme mass coral bleaching. Although most corals at these sites were bleached or pale during our survey, many of the fully bleached corals subsequently died. Although records from 1998 are limited, the 2024 event appears to represent the most severe and widespread bleaching event ever documented in the non-reef temperate regions of Japan.

In the present study, however, no clear correlation was found between bleaching prevalence and DHW among the study sites (p = 0.42, Fig. S2). One possible explanation is variation in coral community composition and species-specific thermal tolerance among sites. Similar to the subtropical sites, severe bleaching was particularly evident in Acropora species. Interestingly, poleward-expanding tropical species (e.g. A. hyacinthus complex, A. solitaryensis and A. muricata), temperate species (e.g. A. cf. glauca), and temperate-endemic species (A. pruinosa) were highly vulnerable to heat stress, with nearly 100% of corals becoming fully bleached. For example, the A. hyacinthus complex, which accounted for approximately 80% of the total cover at Tatsukushi, was almost entirely pale, bleached, or dead. Although this species currently dominates the site, it was not recorded in surveys conducted during the 1930s, suggesting its recent establishment through poleward range expansion29. These findings indicate that poleward-expanding tropical Acropora species are also highly susceptible to extreme thermal stress. In contrast, some species, such as Lithophyllon undulatum, Leptastrea aff. pruinosa and Cyphastrea serailia, which are widely distributed from tropical to temperate regions, exhibited higher tolerance (Fig. 7). However, responses were not uniform. For example, L. undulatum, previously reported as susceptible during the 2016 heat stress event in Singapore46, showed tolerance in this study. Additionally, Acropora species including A. cf. glauca and A. solitaryensis were reported to be resistant to heat stress during the 2016 mass bleaching event at high-latitude eastern Australia47, whereas they exhibited high susceptibility in the present study. It should be noted, however, that the 2016 bleaching event in eastern Australia was relatively mild (DHW 4–9 °C-weeks) compared with the present event. These findings further suggest that although high-latitude coral communities have been proposed to be better adapted to climate change because they experience highly variable environmental conditions48 and may therefore function as climate refugia49, temperate corals are unlikely to remain insulated from the impacts of extreme marine heatwaves.

Moreover, the bleaching susceptibility of species such as Dipsastraea speciosa and the A. hyacinthus complex varied among the present studied sites. In contrast to Tatsukushi site, at Kushimoto, where the A. hyacinthus complex also dominates, bleaching impacts were less severe, and no dead corals were observed during the present survey (Fig. 6, 7). Furthermore, our observations suggest that many bleached corals subsequently recovered at Kushimoto. These differences may be related to the duration and timing of thermal stress. For example, at Tatsukushi, DHW exceeded 9 °C-weeks from early September (10 September) and persisted for 47 days, whereas at Kushimoto, it exceeded 9 °C-weeks approximately two weeks later (21 September) and persisted for 36 days. Additionally, a typhoon passed along the Japanese coast from 18 to 21 September, causing a decline in SST (Fig. S1), which may have alleviated thermal stress and thereby reduced bleaching impacts at Kushimoto.

Population genetic studies have suggested the presence of cryptic species within the A. hyacinthus complex, indicating that pre-existing, recently expanded, and subtropical populations may represent distinct lineages50. Differences in thermal tolerance among cryptic species or local populations may therefore contribute to variation in bleaching prevalence among sites. Additionally, studies of Pocillopora spp. have shown that both host haplotype diversity and symbiont (Symbiodiniaceae) diversity decrease with latitude along the Japanese coast51. Reduced genetic diversity in both host and symbiont communities at the peripheral edge of the species’ distribution may also influence thermal tolerance under heat stress. These patterns may reflect variation in local environmental conditions, but they may also indicate differences in thermal tolerance among local populations. Tateyama was the only site among the eight studied where no coral bleaching was observed (Fig. 6). Although the DHW in 2024 (12.9 °C-weeks) was the highest recorded in the past 40 years, the MMM at Tateyama (27.86 °C) was the lowest among all sites (Figs. 3, 4). Our experimental studies using acroporids (A. pruinosa and A. cf. glauca) from Tateyama showed no bleaching at 28 °C (unpublished data), suggesting that summer temperatures in 2024 remained below the local bleaching threshold. Nevertheless, previous studies suggest that coral bleaching thresholds decrease with increasing latitude, with thermal bleaching thresholds reported to be approximately 26–27 °C for coral communities located around 31°N or 31°S52,53. In addition, it is possible that the coral species dominating this site (e.g. Hydnophora exesa and Leptoseris mycetoseroides) are relatively tolerant to heat stress. Furthermore, local environmental factors such as light intensity and zooplankton availability, which are known to influence bleaching severity and post-bleaching mortality54,55,56, may have contributed to the observed patterns. These results highlight the importance of local environmental conditions and suggest that DHW alone may not reliably predict bleaching severity in temperate regions.

Overall, the extreme SSTs recorded in 2024 caused widespread mass coral bleaching across subtropical and temperate regions along the Japanese coast, affecting tropical, temperate-endemic, and poleward-expanding corals alike. While poleward range expansion of tropical species has been widely interpreted as a mechanism for thermal escape and temporary establishment at higher latitudes29,32,33 our findings demonstrate that such expansion does not confer resilience to extreme heat stress. This vulnerability likely reflects the predominance of heat-sensitive Acropora species among poleward-expanding taxa, together with reduced genetic diversity in high-latitude marginal populations that may constrain thermal tolerance. Collectively, these findings challenge the assumption that temperate regions can function as effective long-term climate refugia32,33 and suggest that ongoing ocean warming may be outpacing the capacity of corals to adapt or shift their distributions.