This species is widespread and rare. 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 moderately 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 <25% 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 less than 25% over the next three generations, or by 2050. Therefore, this species is assessed as Least Concern.

This species is distributed from southern Japan, East China Sea, central Indo-Pacific, oceanic West Pacific, the Solomon Islands and the Hawaiian Islands. It has also been confirmed from southern Vietnam (DeVantier and Turak 2017).

The depth range is 5–54 m, but the species primarily occurs from 5–40 m (Muir and Pichon 2019, Turak and DeVantier 2019, L. DeVantier pers. comm. 2024).

This species is rare (DeVantier and Turak 2017), but may form local aggregations on suitable soft substrates near the reef via asexual budding of new individuals (Hoeksema 2015).

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 (Hughes et al. 2018). 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 poorly understood and varies by species and locality (van Hooidonk et al. 2013, Logan et al. 2014). 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 1°C of adaptation is lower given this species is not primarily restricted to depths shallower than 30 m and is moderately 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 understanding that a coral species with shallow depth preferences is more frequently exposed to extreme temperatures and is expected to decline at a faster global rate than species that also or primarily occur in deeper, cooler waters (Riegl and Piller 2003). 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 future global level decline of less than 25% by the year 2050, 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’ entire 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 5-54 m and is predominately found at depths greater than 10 m, generalized abundance is considered rare, overall population is not restricted or highly fragmented, does occur off-reef, is more resilient to disease, does not recover well from bleaching or disease, has a low susceptibility to crown-of-thorns starfish, is moderately susceptible to bleaching, has an unknown 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 inferred to be moderately susceptible to threats related to reef degradation. Therefore, past decline was suspected from the 50% percentile of estimated coral cover loss, resulting in a global level decline of <25% since 1989, or over the past three generations.

This species is found on soft substrata on reef slopes between 10-25 m depth. It is a single free-living polyp with a maximum size of 8.5 cm diameter. The species can reproduce asexually by budding or sexually releasing larvae, which settle as juveniles attached to hard substrate before detaching as free-living adults (Hoeksema 2015).  

While there is some information regarding the age in which corals reach sexual maturation, it is largely based on measurements of size as a proxy for age (Harrison and Wallace 1990), 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). Therefore, we assume that the average age of mature individuals on a given reef is greater than eight years. 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 likely to be more than ten years. Therefore, any population decline rates for the Red List assessment are measured over at least 30 years.

This species is moderately susceptible to bleaching and has a low susceptibility to disease. 

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). 

Coral reefs are threatened by human and natural stressors at a range of scales. In general, the greatest large-scale threat to corals is from global climatic change, which is linked to lethal seawater temperature anomalies, along with increased frequency and severity of El Niño Southern Oscillation (ENSO) events and storms, and ocean acidification (Pandolfi et al. 2011, IPCC 2018), each a major threat to reefs in their own right. The most recent, and first, multi-year, global ‘bleaching’ event (spanning hundreds of kilometres or more) was from 2014 to 2017. Globally, 75% of reefs were affected by bleaching-level stress, with more than 50% of affected reef areas impacted at least twice over the period (Hughes et al. 2018, Blunden et al. 2018, Eakin et al. 2019), and some localities experienced almost complete coral cover loss (Vargas-Ángel et al. 2019). The first global coral bleaching event was in 1997-98, however this had also been preceded by multiple smaller regional and local scale bleaching events since at least 1982 (Goreau et al. 2000). While coral populations can be resilient to coral bleaching and bounce back (e.g. Diaz-Pulido et al. 2009, Pisapia et al. 2016), more frequent bleaching events in the future are expected to prevent full reef recovery and cause local extinctions of some species (van Hooidonk et al. 2016, Sheppard et al. 2020). 

Localised threats to corals include over-intensive fisheries, coastal development (industry, settlement, tourism, and transportation), changes in native species dynamics (competitors, predators, pathogens and parasites), introduction of invasive species (competitors, predators, pathogens and parasites), destructive fishing (e.g. using dynamite), chemical fishing, pollution from agriculture and industry, domestic pollution, and recreation and tourism activities  and global trade (Burke et al. 2012). Some of these threats impact corals directly, such as being physically disturbed and smothered with sediment during a construction project (Erftemeijer et al. 2012), while others operate indirectly via ecosystem processes and linkages between corals and other reef organisms. Macroalgae is major competitor with corals that reduces growth, causes disease, prevents new coral recruitment and can tip the entire ecosystem into a less diverse and less productive ‘algal-dominated’ reef (Hughes 1994, Bellwood et al. 2004). Macroalgal levels are controlled by both bottom-up provision of nutrients (Fabricius 2005), and top-down herbivory by parrotfish (Mumby et al. 2007), hence while the immediate threat to the coral is be the algae, the ultimate threat may be sewage, fertiliser from agriculture or overfishing of herbivorous fish. The complex nature of the coral reef ecosystem means that while the immediate threat may be obvious (e.g. macroalgae, disease, crown-of-thorns outbreak), the ultimate threat is often less clear (Nyström et al. 2008, Anthony et al. 2015).

Coral disease has emerged as a serious threat to coral reefs worldwide, a major cause of reef deterioration (Weil 2006), and may be as likely to cause mortality as bleaching in the coming decades (Maynard et al. 2015). In addition to thermal stress, increased coral disease prevalence and mortality can be linked to reduced water quality and clarity, dredging associated sedimentation and plastic pollution (e.g. van Woesik and McCaffrey 2017). Based on a survey of 159 reefs in the Asia-Pacific region, the likelihood of disease increased 20-fold when corals are in contact with plastic (Lamb et al. 2018). In the Indo-Pacific, disease outbreaks have also been reported from the Great Barrier Reef (Willis et al. 2004), Marshall Islands (Jacobson 2006) and the Northwestern Hawaiian Islands (Aeby et al. 2006). Increased coral disease levels on the GBR were correlated with increased ocean temperatures (Boyett et al. 2007), supporting the prediction that disease levels will be increasing with higher sea surface temperatures.

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 overlaps with 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.