Justification
This species has a restricted range with the majority of its known population occurring only in the Chagos Archipelago, but with a record on Île du Nord, Saint-Brandon in 2010, 1,000 km to the southwest. If this sighting is excluded, the estimated area of occupancy (AOO) is inferred to be less than 2,000 km2 but greater than 10 km2. This species has shown severe population fluctuations as a result of mass coral mortality across its known range. It was common in the 1970s, has undergone two major population crashes (in 1998 and 2015/2016) with intervening recovery, and is now rare and susceptible to bleaching and extensive reduction of coral reef habitat. Based on long-term monitoring of Chagos Archipelago reefs, this species has declined by more than 99% since 1998, or over the past ~25-30 years (three generations) likely due to severe warming events, especially following the 2015/6 back-to-back bleaching events; by 2017, the population had declined so severely that no live individuals were seen in dedicated searches that year, or the following one. In 2020 and 2021, some limited recovery has been observed, mirroring the general recovery in regional coral cover after the 1998 mortality event. Future repeated and increasingly frequent mass bleaching events (in combination with extremely low local abundances confounding sexual hybridization opportunities) pose a high risk of causing the extinction of this species over its entire range. It is listed as Critically Endangered A2bc.
Geographic Range Information
Despite several historic observations of this species in the wider Western Indian Ocean, the only confirmed sightings for over a decade have been in the Chagos Archipelago. It is considered to have a restricted distribution with the majority of its global population occurring only in the Chagos Archipelago (Obura 2012, Sheppard et al. 2020). Within the Chagos Archipelago, recent sightings (since 2019) have been confined to reefs at a limited number of islands (B. Wilson pers. comm. 2024), including Moresby Island and Ile Parasol (Peros Banhos Atoll), Ile de la Passe (Salomon Atoll), Middle Brother (Great Chagos Bank), Egmont Islands and Nelson Island (R. Dowell pers. comm. 2024) and Diego Garcia. Several photographs of multiple (approximately twenty) individuals taken in 2010 of a small population in 1 m depth in rocky-platform seagrass bed (Île du Nord) on Saint Brandon, documents a location 1,000 km southwest of Chagos (Obura 2012, D. Obura pers. comm. 2024). It is unknown if this population is or was a small outlier from the main distribution in the Chagos Archipelago (Sheppard et al. 2020), or if the species is widespread across the intervening banks with suitable habitat (Saya de Malha, Saint-Brandon, Nazareth Banks). Searches of these banks during an expedition in 2022 were inconclusive as appropriate habitats, and the Île du Nord population at Saint Brandon, were not visited (B. Wilson pers. comm. 2024). It has not been found at the much closer Addu Atoll in the Maldives 400 km north of Chagos, and despite prolonged surveys in the Seychelles, Mauritius and Réunion, and some mis-identified records, it was not found in those localities either (Wells and Davies 1966, Sheppard et al. 1983, Obura 2012, Sheppard et al. 2020).
The depth range is 1-45 m but primarily occurs no deeper than 25-30 m (Sheppard et al. 1983, Sheppard et al. 2020, B. Wilson pers. comm. 2024); recent ROV surveys down to 90 m have not recorded any colonies below these shallow depths (C. Diaz pers. comm. 2024).
The estimated extent of occurrence (EOO) is larger than 55,000 km2. According to the 2018 UNEP-WCMC global coral reef distribution map, the area of coral reef from 0-30 m depth within the range of this species, is 1,771-2,017 km2. Due to its current rarity, the area of occupancy (AOO) is inferred to be less than 2,000 km2 but greater than 10 km2. The number of locations is one based on the major threat from severe coral bleaching events which are increasing in frequency as climate change progresses and acts on the entirety of this species' range.
Population Information
This species is currently rare but was one of the twenty-five most common corals in the Chagos Archipelago in the 1970s (Sheppard
et al. 1983, Sheppard
et al. 2020). Based on long-term monitoring of Chagos Archipelago reefs conducted since that period, severe declines of coral cover were observed over time, including local extirpation of some species. Specifically, total coral cover averaged 65% in 1979 and 10% in 2019, which represents an 85% decline over that 40-year period. This decline is due to two sharp mortality events in 1998 and 2015/16 (which were two consecutive years of above average temperatures), with intervening recovery from 1998-2015, and initial recovery after 2016. The mass mortality of corals in 1998 and 2015/6, is attributed to severe El Niño and Indian Ocean Dipole events, which cause extended periods of high water temperatures. The trajectory of Chagos Archipelago corals after the mortality events has been well documented, including initially-low coral settlement and reduced habitat quality available for settlement for several years after the mortality events (Sheppard
et al. 2002, 2020).
This species has shown significant population fluctuations resulting from thermal stress and the consequent mass mortality of corals. It became much less common after the 1998 warming event, became reasonably common again in 2006 (D. Obura pers. comm. 2024) and up to 2015, then almost disappeared after the 2015 warming event (C. Sheppard pers. comm. 2024). With dedicated surveys since 2015, the slow initial population recovery can be shown:
- In 2017, only dead skeletons of this species were sighted, and in 2018, a few remnants of live and growing polyps were seen at one site each at Salomon Atoll and Peros Banhos Atoll (Sheppard et al. 2020).
- In 2019, the species was again only observed at two sites, and in extremely limited numbers, with four specimens being spotted at Moresby Island (Peros Banhos Atoll) and two at Île de la Passe (Salomon Atoll); of these, all but one of the specimens consisted of a remnant patch of tissue that had survived on a largely dead colony, and mostly on shaded sides (B. Wilson pers. comm. 2024). All surveys during these years covered an area where previously, there had been hundreds of colonies of this species.
- In early 2020, limited surveys (due to the outbreak of the global pandemic) discovered a single large (approximately 40 cm in diameter) healthy colony at Barton Point on the seaward side of the northernmost tip of Diego Garcia. More significantly, fifteen healthy and mature colonies were found clustered around a single knoll in the small lagoon at Middle Brother, the largest group of proximal colonies discovered in over a decade.
- In March and April 2021, four colonies (three healthy mature ones and a remnant fragment of a larger colony) were spotted over two dives at Île de la Passe (Salomon Atoll); ten colonies on a single dive at Moresby Island (Peros Banhos Atoll); and a single colony at Île Parasol (Peros Banhos Atoll), which was the largest one seen in over a decade at almost 1 m in diameter (N. Dunn pers. comm. 2024). In the sheltered lagoon at Middle Brother, however, over forty healthy colonies were observed, including the discovery of a juvenile, likely less than a year old and the only one recorded in three years of expeditions, indicating the species continues to reproduce and settle, albeit with limited success. A single dive at Barton Point (Diego Garcia) also recorded five large mature colonies (B. Wilson pers. comm. 2024).
- In July and August 2023, an expedition to comprehensively survey the reefs of SW Diego Garcia and the lagoon at Middle Brother, recorded twenty and one hundred and one adult colonies, respectively. A subsequent expedition surveying the reefs of E Diego Garcia, also reported >20 colonies (K. Dawson pers. comm. 2024).
These repeated population collapses and initially slow recovery, show the high vulnerability of the species to thermal stress from global warming. Its predominantly shallow depth range suggests the species is physiologically adapted to shallower waters (Sheppard
et al. 1983) and may thus have higher resistance to thermal stress, but it also means it may not benefit from the refugia of deeper and cooler waters. Local populations may be highly vulnerable to local extirpation, and the highly restricted global distribution makes the species vulnerable to individual thermal stress events. Both the increasing severity of warming events and their increasing frequency (the species took almost ten years from 1999 to 2006 to show much recovery from the 1998 bleaching event) pose an increasing risk to the species. The species has been considered ecologically and functionally extinct in the Chagos Archipelago in 2019 (Sheppard
et al. 2020), though rarity is a characteristic of approximately a third of coral species at many locations (Huang 2012, Foden
et al. 2013). Repeated mass coral bleaching events at successively shorter intervals to ten years and less may make the global extinction of the species increasingly likely.
Habitat and Ecology Information
This species is found on reef slopes and lagoons in clear water (Sheppard et al. 1983, 2020). Recent annual expeditions to the Chagos Archipelago (2019-2023) confirm that the species is most common in shallow waters down to 18 m, and currently occurs in a mixture of high-energy ocean-facing reefs (Moresby Island, Ile de la Passe, and Diego Garcia) and lagoonal reefs (Middle Brother) (B. Wilson pers. comm. 2024). The small lagoon at Middle Brother is of particular significance, harboring the largest single population of neighboring colonies of this species in the Chagos Archipelago and it is suspected that limited water exchange (and therefore dispersal of larvae) between the lagoon and the surrounding ocean might be contributing to its local success here – there is however a risk of this population suffering from genetic drift, with potential impacts on their fitness. The water temperature of lagoons in the Chagos Archipelago are consistently found to be higher than surrounding seaward reefs (Sheppard et al. 2017) and so the colonies within Middle Brother may potentially have become adapted to higher sea temperatures, and able to better survive recent warming events. The exposed reefs of the southernmost island (Diego Garcia) also host significantly higher numbers of colonies than elsewhere in the Archipelago; however, none have been observed within the lagoon there (B. Wilson pers. comm. 2024).
The age at first maturity of most reef-building corals is typically three to eight years (Wallace 1999). Based on this, we infer that the average age of mature individuals of this species is greater than eight years. Based on average sizes and growth rates, we also infer that the average length of one generation is 10 years. Longevity is not known, but for the large corals observed up to the bleaching events of 2015/16 (C. Sheppard and D. Obura pers. comm. 2024), was substantially greater than 10 years, and likely multiple decades. Therefore, any population decline rates estimated for the purposes of this Red List assessment are measured over a time period of 30 years.
Threats Information
The remote and predominantly uninhabited Chagos Archipelago, where this species is suspected to be endemic, is protected from the majority of direct anthropogenic pressures, such as pollution, overfishing and coastal development, both by its extreme isolation from major human populations and its no-take MPA status (IUCN Category 1) since 2010. Nevertheless, like other reef systems in the Western Indian Ocean, the Archipelago has experienced severe bleaching events in 1998 and the double dipole event of 2015/2016, which dramatically reduced coral cover, including the populations of this species (Head et al. 2019, Sheppard et al. 2020). While Chagos Archipelago reefs showed high levels of coral recovery following the 1998 bleaching event, there was a period of 2-3 years in which recovery was minimal and scientists feared recovery could fail (Sheppard et al. 2002), followed by a rapid increase in coral cover (documented in 2006) to the mass mortality in 2015. With repeated mortality in 2015/16, a similar pattern of delayed-then-rapid recovery may be expected (due to the lack of other threats over most of the archipelago) until the next severe warming event (potentially the 2024 El Niño). It is likely that this will recur at a shorter interval than for the period 1998-2015 (17 years) before a third rapid decline in corals recurs. Repeated ratcheting downwards of coral communities in the Chagos Archipelago are expected to continue due to increasing frequency of severe marine heatwaves, and shorter periods of recovery between these events.
The atoll Diego Garcia hosts a large military base with significant land-use change, lagoon-alteration and pollution threats that may impact on surrounding reefs directly, and through interactions with humans, e.g., coral diseases. Other reef areas in the Chagos Archipelago have had no local or resident human populations since 1973, and so consequently, fishing has been significantly reduced, particularly with the declaration of the no-use MPA in 2010; however, there are substantial on-going issues with IUU fishing activity in the wider Archipelago that include reef-based bony fish and shark species.
Use and Trade Information
Conservation Actions Information
All stony corals are listed on CITES Appendix II. The reefs of the Chagos Archipelago are protected and monitored by the UK government (Sheppard et al. 2020).
Continued protection of Chagos Archipelago reefs, the full currently-confirmed extent of the species, from local disturbances, and limiting global warming to the maximum extent possible, are likely the single most important actions to minimize the risk of extinction of this species.
The historic and potential range for the species extends across the banks shared by Seychelles and Mauritius – Saya de Malha, Nazareth bank and Saint Brandon. Field surveys to confirm its presence/absence across this range are of the highest priority, for incorporation into national marine spatial planning and conservation processes in the two countries, and their shared jurisdiction over the Saya de Malha bank.
The species is in a mono-specific genus and thought to be a relict species of Tethyan corals most closely related morphologically to Atlantic species in the Meandrinidae family (Obura 2012, 2016), although molecular analyses have placed this coral within the family Euphylliidae (Fukami et al. 2008). As such, its phylogenetic uniqueness is very high, corresponding to the EDGE species concept (Isaac et al. 2007).
The genome and transcriptome for a number of individuals has been sequenced and is currently being assembled and annotated, with hopes of addressing the current paucity of knowledge of the biology and ecology of this species (B. Wilson pers. comm. 2024). An Emergency Species Recovery plan for its continued preservation was also implemented in July 2023, with twelve live colonies (six each from Middle Brother Lagoon and SW Diego Garcia) collected and transported to the UK, to ensure that a viable live ex situ population is maintained in the event of further extreme coral mortality events in the region (B. Wilson pers. comm. 2024). These colonies have been successfully fragmented, with live fragments being shared with several institutions for conservation redundancy and security of the ex situ population; in March 2024, captive colonies also sexually reproduced and recruited to settlement tiles (J. Craggs pers. comm. 2024). Tissue samples and gametes will also concurrently be taken for cryo-preservation. These measures are of the utmost urgency, such that should the coral become Extinct in the Wild, a founder population surviving ex situ might be used to repopulate reefs in the Chagos Archipelago, and potentially others in its historic range.
