2. Biodiversity

The Region is home to highly diverse coral assemblages, which incorporate all forms of corals belonging to the two subclasses Hexacorallia and Octocorallia (octocorals). Hexacorallia includes reef-building corals, sea anemones and black corals, and Octocorallia encompasses soft corals, gorgonians and sea pens.^289,290

Reef-building corals build hard skeletons by depositing calcium carbonate ²⁹⁰ and often have symbioses with dinoflagellate algae called zooxanthellae.²⁹¹ These zooxanthellate corals are major habitat-forming organisms on the Reef,²⁹⁰ and they exhibit a high degree of morphological, functional and genetic diversity.^21,292 Morphological diversity is evidence in the variety of growth forms, such as massive, encrusting, plating, branching, and solitary free-living corals.²⁹⁰ Branching corals, including tabular forms,^293,294 are often represented by fast-growing species that are key for the function of the Reef.²⁹⁵ These corals are sensitive to disturbances, such as thermal stress ²⁹⁶, cyclones and outbreaks of crown‑of‑thorns starfish,²⁹⁷ but they are also first and fastest to recover and provide essential habitat for other species.²⁹³ Diversity assessments in the Region often group corals into genera or families (rather than looking at the species level).^185,277 This approach underestimates the diversity of corals, particularly if grouping species belonging to the genus Acropora, which is the most species-rich and numerically abundant coral genus in the Indo-Pacific region.²⁹²

Black corals, sea anemones and octocorals are diverse groups of reef organisms, and they occupy diverse habitats and microhabitats across the Region.^289,298 These groups are critical for providing habitat complexity.²⁹⁸ However, comprehensive taxonomic inventories are limited (and genetic analyses of sea anemones and octocorals are lacking), which leaves major knowledge gaps with respect to their diversity and distribution. Black corals are ecologically important because of their extensive distributional range. They provide habitat to diverse invertebrate communities from just below the surface to a depth of more than 8000 metres,²⁹⁹ but their diversity and distribution are poorly understood.³⁰⁰ However, limited genomic analysis has seen the Region’s known black coral species increase from 7 to 24.^299,301

Sea anemones are a prominent member of coral reef communities and can establish symbiosis with zooxanthellae.³⁰² Overall, it is estimated that 40 morpho-species of anemones occur in the Region.²⁸⁹ A total of 37 species of sea anemones have been described in submerged reefs at depths between 50 and 65 metres.³⁰³

The octocorals include species that have no solid calcium carbonate skeleton.²⁹⁸ This group includes soft corals, gorgonians and sea pens. Octocorals are a common feature of benthic habitats in shallow areas of the Reef and become increasingly prevalent as depth increases.³⁰⁴ An estimated 289 species of soft corals have been identified in the Region using traditional taxonomic methods.²⁹⁸ Soft corals provide structural complexity in the benthos across reef habitats, and essential habitat, shelter and food for many reef species.³⁰⁵ Monitoring of soft corals in the Region occurs as part of broader monitoring programs focusing on reef-building corals but summary soft coral-specific analyses are limited.^190,277

Community composition in corals is strongly influenced by cross-shelf, latitudinal and depth gradients.³⁰⁶ Shallower areas (less than 30 metres) of the Reef are often dominated by zooxanthellate corals. As light availability diminishes towards deeper areas, communities become increasingly dominated by corals without these symbiotic partners.³⁰⁷ Such corals tend to be solitary and to live in low light areas of shallow reefs; they are common on deeper banks down to the deepest waters of the continental slope (Section 2.3.9).³⁰⁸ However, a few zooxanthellate corals can also live in low-light environments of the mesophotic zone (at depths between 30 and 150 metres), showing that some corals can adapt to marginal and extreme environments.³⁰⁹ The study of corals in these and other extreme environments, such as mangrove lagoons, is becoming increasingly important for understanding how coral species might respond to changing environmental conditions linked to climate change.³¹⁰

Corals continue to be extremely vulnerable to climate change, crown-of-thorns starfish outbreaks and reduced water quality in inshore areas. One of the major threats to zooxanthellate corals is heat stress associated with ocean warming and extreme temperatures,³¹¹ which can cause bleaching and mortality of corals (Section 3.4.6). Mass bleaching occurred in 2020 and 2022, although widespread coral mortality was not observed following either event. Different coral species have varying susceptibility to bleaching; fast-growing, branching species are often more vulnerable than massive species.^296,312 Following the 2020 and 2022 mass bleaching events (Section 2.3.5), most reef-building corals recovered, and widespread loss of diversity was not recorded.^161,182,313 The extent of impacts on other lesser-known species of octocorals, sea anemones and black corals is unknown.

Early reports indicate impacts to branching and plate coral species occurred in localised areas following cyclone Kirrily in January 2024,¹⁸⁹ although a full assessment has not yet been made. Similarly, analyses over time will shed light on the extent of mortality and other impacts from high temperatures and subsequent extensive bleaching in summer 2023–24.^167,314 Systemic monitoring and accurately identifying and distinguishing coral species can enable the assessment of coral condition, particularly in relation to the functional traits that support resilience. The effects of cumulative pressures on species diversity are currently a knowledge gap.

Relatively few acute disturbances have occurred since 2019, facilitating recovery of some fast-growing coral species. Increases in cover of these species contribute to some improvement in condition of corals overall. Nevertheless, recovery across other species is less certain, and overall coral diversity likely remains affected. Crown-of-thorns starfish and reduced water quality have also affected corals in some locations. While widespread mortality or loss of diversity after the 2020 and 2022 mass coral bleaching events was not recorded, corals remain extremely vulnerable to rising ocean temperatures.