This species is widespread and uncommon. Global level, species-specific population data are limited; however, coral reefs have declined globally and are expected to continue rapidly declining due to increasing severe bleaching conditions under temperature stress caused by climate change as well as a variety of other threats. Our species-specific vulnerability traits analysis indicates this species is highly susceptible to major threats related to coral reef degradation (e.g., disease and bleaching). We applied two analytical approaches involving two different global coral datasets and the species’ distribution map as proxies to infer population decline. Based on global coral cover monitoring data, this species experienced a suspected decline of about 12% over the past three generations, or since 1989. Based on the projected onset of annual severe bleaching (ASB) conditions via both SSP2-4.5 and SSP5-8.5 scenarios of global climate model data, in combination with the species’ depth range, distribution and bleaching vulnerability, this species is suspected to decline by at least 62% over the next three generations, or by 2050. Since the species qualifies for a higher category under the projected decline, we therefore list it as Endangered A3ce. The change in status from the previous assessment reflects updated declines calculated from improved data on modeled coral cover loss and projected date of annual severe bleaching, along with improved knowledge of species traits.

This species is found in the southern Red Sea, the Gulf of Aden, the southwest and northern Indian Ocean, the central Indo-Pacific, Australia, Southeast Asia, Japan, the East China Sea, the oceanic west Pacific, and the Line Islands (Wallace 1999, Wallace et al. 2012). It has also been confirmed from eastern Thailand, northern Vietnam and Taiwan (Huang et al. 2015).

The depth range is 3-35 m, but the species is most common at 5-25 m (L. DeVantier pers. comm. 2024).

This species is uncommon (DeVantier and Turak 2017).

Species-specific, global level population information is limited. However, coral reefs are experiencing severe global level declines due to increasing water temperatures caused by climate change (Hoegh-Guldberg et al. 2017, Hughes et al. 2018, Donovan et al. 2021). For the purposes of this Red List assessment, we used species-specific vulnerability traits and two analytical approaches based on two global coral datasets to infer past (GCRMN 2021) and future (UNEP 2020) population trends. 
   
Approach 1: Future population trend
The projected onset of annual severe bleaching (ASB) was applied as a proxy to estimate global level population decline. ASB represents the date at which a coral reef will likely experience severe bleaching conditions annually, and beyond which the species will experience a greater than 80% decline as it is not expected to recover (van Hooidonk et al. 2014). ASB is defined as at least eight Degree Heating Weeks (DHW) occurring over a three-month period within a year, and where a DHW occurs when the sea surface temperature is at least 1°C above the maximum monthly mean (van Hooidonk et al. 2014; 2015). We defined the onset of ASB as corresponding to 80% or more decline, however, this is conservative as other studies have found that coral populations may experience near complete mortality and are unlikely to recover with just two incidences of ASB per decade (Obura et al. 2022).

To calculate ASB for each species we applied spatial data made publicly available via a United Nations Environment Programme report (UNEP 2020) that used the 2019 IPCC CMIP6 global climate models to estimate the projected onset of ASB for the years 2015-2100 on a 27 km x 27 km grid according to the 2018 WCMC-UNEP global coral reef distribution map, which has a resolution to 30 m depth. These data are available via two scenarios of Shared Socioeconomic Pathways (SSP), with SSP5-8.5 representing current global emissions and SSP2-4.5 representing a future reduction in emissions (UNEP 2020). We applied SSP5-8.5 since it follows the precautionary approach recommended by the IUCN Red List methodology and SSP2-4.5 since it represents a more moderate climate change scenario that better tracks current policy projections (Roelfsema et al. 2020, Obura et al. 2022). To acknowledge varying levels of coral adaptation to thermal stress, both of these spatial data layers are available for all quarter degree intervals between 0° and 2°C (UNEP 2020); however, coral adaptation in general is little understood and varies by species and locality (Bay et al. 2017, Matz et al. 2020, Logan et al. 2021). To account for adaptation, we calculated two estimates of ASB onset for both the SSP5-8.5 and the SSP2-4.5, where the first estimate assumes the species has no level of adaptation (0°C) and the second assumes a capacity for 1°C of adaptation. We clipped each of these four UNEP (2020) spatial data layers to the species’ distribution and calculated the average year of ASB onset across all overlapping grid cells. 

Based on this spatial analysis, the onset of ASB across this species’ range is projected to occur on average by the year 2035 for SSP5-8.5 and by 2038 for SSP2-4.5 assuming no level of adaptation and by the year 2062 for SSP5-8.5 and by 2071 for SSP2-4.5 assuming 1°C of adaptation. For species where ASB occurs within 3-generation lengths, the 3-generation reduction is calculated as 80% multiplied by two proportions: (i) the proportion of the species' depth range that is in 0–30 m range, and (ii) for widespread species, the proportion of cells within the species' range that are expected to experience ASB under SSP2-4.5 before 2050 (three generation lengths). We inferred that the uncertainty associated with the estimate of population decline based on no level of adaptation is lower given this species is primarily restricted to depths shallower than 30 m and is highly susceptible to bleaching. For widespread species, the final estimate of decline was further adjusted by excluding the proportion of cells within its range that were expected to experience ASB under SSP2-4.5 after 2050 (three generation lengths), in order to account for the potential resilience of species to the asynchronous variability of bleaching events that occur across the Indo-Pacific. The relative vulnerability to bleaching (i.e., highly susceptible, moderately susceptible, or more resilient) is primarily based on scientific species expert knowledge. The application of the species’ depth range as a vulnerability factor is based on the assumption that a coral species with shallow depth preferences is more frequently exposed to extreme temperatures and might decline at a faster rate in some places than species that also occur in deeper, cooler waters (Riegl and Piller 2003), although this is not always the case (e.g., Smith et al. 2016, Frade et al. 2018). Ocean acidification, which is measured by aragonite saturation, is also considered a major threat to corals due to the impacts of climate change, however, the impacts are expected to be more severe in cooler and/or deeper waters (Couce et al. 2013, van Hooidonk et al. 2014, Hoegh-Guldberg et al. 2017). Although the exact threshold of aragonite saturation that is expected to cause significant decline is not well-known, in the Pacific, changes in aragonite saturation are expected to be most severe in high-latitude reefs (van Hooidonk et al. 2014). Therefore, this species is suspected to experience a projected global level decline of at least 62% by the year 2050, or three generations in the future, regardless of the SSP2-4.5 or SSP5-8.5 scenario. 

Approach 2: Past population trend
Coral reef monitoring data were also applied as a proxy to estimate global level population decline. The Global Coral Reef Monitoring Network (GCRMN) compiled data related to the status and trends of coral reefs in 10 regions from 1978-2019 via the scientific monitoring observations of more than 300 network members located throughout the world. We applied the publicly available data on estimations of the percent of live hard coral cover loss at the 20%, 50% and 80% confidence intervals in the 37 subregions of the Indo-Pacific (GCRMN 2021) to estimate species population decline over the past three generations (1989-2019). The proportion of the species’ range that overlapped with each of the subregions was estimated using the Red List distribution map. The sum of the proportion of the subregional species distribution multiplied by the percent of coral cover loss in each subregion was then used to calculate the 20%, 50% and 80% estimates of coral loss across this species’ range.

To inform the choice of the best (i.e., lowest level of uncertainty) out of the three percentile declines, we considered 11 species-specific traits related to vulnerability to coral cover loss. Given this species’ depth range is 3-35 m and is predominately found at depths greater than 10 m, generalized abundance is considered uncommon, overall population is not restricted or highly fragmented, does not occur off-reef, is highly susceptible to disease, does not recover well from bleaching or disease, has a high susceptibility to crown-of-thorns starfish, is highly susceptible to bleaching, has a relatively higher susceptibility to the impacts of ocean acidification (Kornder et al. 2018), did not have >10,000 pieces exported annually in the aquarium trade between 2010-2019, it is overall suspected to be highly susceptible to threats related to reef degradation. Therefore, past decline was inferred from the 80% percentile of estimated coral cover loss, resulting in a suspected global level decline of 12% since 1989, or over the past three generations.

This species occurs in shallow, tropical reef environments in a wide variety of environments, especially upper reef slopes and lagoons (Veron et al. 2016). It occurs only subtidally, usually in protected waters and is most commonly encountered in enclosed lagoons or sheltered slopes with sandy substrata (Wallace 1999). 

While there is some information regarding the age at which corals reach sexual maturation, it is largely based on measurements of size as a proxy for age (Harrison and Wallace 1990, Rapuano et al. 2023), which can be problematic in modular animals because of processes such as partial mortality and fission (Hughes and Jackson 1980). Nonetheless, it appears that many brooding coral species tend to reach puberty at about 1-2 years of age, which is much earlier than many broadcast-spawners that become reproductive at 3-8 years or more (Harrison and Wallace 1990, Wallace 1999, Rapuano et al. 2023). The age at first maturity of most Acropora species is typically 4 years; however, it can vary between 3 and 8 years (Harrison and Wallace 1990, Iwao et al. 2010, Baria et al. 2012, Montoya-Maya et al. 2014, Ligson and Cabaitan 2021). Furthermore, based on average sizes and growth rates, we assume that the average generation length is 10 years, unless otherwise stated. Total longevity is not known for any coral, but is likely to be more than ten years. Therefore, any population decline rates for the Red List assessment are measured over at least 30 years.

Many members of this genus have a low resistance and low tolerance to bleaching and disease, and are slow to recover. Two different diseases have been recorded for this species (Willis et al. 2004). Bleached colonies of this species were observed in Andaman (India) during the 2010 bleaching event (Krishnan et al. 2011). Extensive mortality, including of this species, occurred in the Andaman Sea as a result of the 1995 bleaching event (Phongsuwan 1998). This species frequently bleached off Kenting, Taiwan in 1987 (Ho 2003). This species was amongst the most severely bleached during the 2015-2016 bleaching event off Lizard Island, Great Barrier Reef (Hoogenboom et al. 2017). This species is particularly susceptible to crown-of-thorns (Z. Richards pers. comm. 2008).

The most critical threat for this species, like for most coral species, is the extensive degradation and reduction of coral-reef habitat because of a combination of local and global threats (Hughes et al. 2017, Hoegh-Guldberg et al. 2017, Donovan et al. 2021). The increasing threats from climate change are being further compounded by additional local stressors, such as pollution and overfishing (Knowlton and Jackson 2008, Lamb et al. 2018, MacNeil et al. 2019, Donovan et al. 2021).

Generally, the biggest threat to the persistence of corals is climate change (Hoegh-Guldberg et al. 2017, Hughes et al. 2017, Sully et al. 2019), and more specifically - ocean warming and marine heatwaves that are leading to an increase in the frequency and intensity of events of anomalously high water temperatures (Hoegh-Guldberg et al. 2019, Laufkötter et al. 2020). Under anomalously high temperatures, the symbiotic relationship between corals and their photosynthetic symbionts is disrupted, and many corals begin to bleach (Glynn 1996, Hoegh-Guldberg 1999, Warner et al. 1999, Loya et al. 2001, van Woesik et al. 2022). Mass bleaching events resulting from thermal stress have become increasingly common in the last two decades and may lead to widespread coral mortality and changes in overall reef community over large areas (Loya et al. 2001, Graham et al. 2015, Hughes et al. 2018, Safaie et al. 2018, Stuart-Smith et al. 2018, McClanahan et al. 2019, Sully et al. 2019).

Superimposed on thermal stress and bleaching are additional stressors that can either directly threaten corals or exacerbate coral mortality after thermal stress (Kennedy et al. 2013, MacNeil et al. 2019, Abelson 2020, Donovan et al. 2021, Knowlton 2021). For example, increasing number of storms per season, overfishing, high levels of nutrients, and other kinds of pollution are steadily increasing in magnitude and threatening coral reefs (Wiedenmann et al. 2013, Zaneveld et al. 2016, MacNeil et al. 2019, Donovan et al. 2020). Moreover, in some localities, increased amounts of outbreaks of the corallivorous sea star, crown of thorns, can cause substantial damage to the reef, contributing to the overall decline and reef destruction (Saponari et al. 2015, Pratchett et al. 2017).

The prevalence of coral disease is also rising (Aronson and Precht 2001, Rosenberg and Loya 2004, Sutherland et al. 2004, Weil et al. 2012, Maynard et al. 2015), especially in the Caribbean (Aronson and Precht 2001, Precht et al. 2016, Aeby et al. 2019, Alvarez-Filip et al. 2019, Muller et al. 2020). The increasing spatial spread and extent of diseases are associated with ocean warming (Muller et al. 2008, Ruiz-Moreno et al. 2012, Randall and van Woesik 2015) and additional anthropogenic stressors (Vega Thurber et al. 2014, Maynard et al. 2015). The escalating impacts of global warming alongside the ongoing increases in local anthropogenic stressors and diseases are causing fundamental changes to coral reefs and place entire reef systems at a high risk of collapse.

All stony corals (Order: Scleractinia) are listed on CITES Appendix II, and under Annex B of the European Union Wildlife Trade Regulations. Moreover, several countries (e.g., India, Israel, Jordan, Djibouti, and the Philippines) at various stages have banned either the trade or the export of CITES II listed species, which includes all stony corals.

Parts of this species’ range are within marine protected areas. 

Recommended measures for conserving this species include research in fields of taxonomy, demography, ecology and habitat status, reproduction and dispersal, threats, resilience, resistance, recovery, restoration actions, establishment and management of new protected areas, expansion of protected areas, recovery management, diseases, and pathogens.