2. Biodiversity

The Region supports a huge diversity of other (non-coral) invertebrates, including small symbiotic anemones,³¹⁵ giant clams,³¹⁶ sponges,³¹⁷ lace-like bryozoans and sediment‑dwelling worms. More than 12,000 different invertebrate species have been described,^313,318 and these ‘other invertebrates’ account for the greatest overall animal diversity and biomass across the Region.³¹⁹ 

Invertebrates occupy the full range of habitats across the Region. They play important roles in ecosystem processes that support functional integrity of coral reefs,³²⁰ including primary production,³¹⁶ nutrient cycling,³¹⁷ calcification,³¹⁶ bioerosion,³²¹ symbioses,^315,322,323 ecosystem engineering ^324,325 and recruitment facilitation.³²¹ Several invertebrates also have cultural and economic importance, for example, as traditional food, in Reef tourism, as part of school educational programs, or as targets for recreational and commercial fisheries.^{326,327,328,329}

Invertebrates are affected by a range of different pressures and stressors, in particular climate change. Ocean warming directly affects survival in many invertebrates, and tropical marine species are often particularly vulnerable as they tend to live closer to their thermal limits.^330,331,332 Sessile benthic invertebrates are directly exposed to repeated thermal stress events, such as those in 2020, 2022, and 2024, and a lack of local thermal refugia in many areas means that only highly mobile species can actively avoid exposure.³³³ The physiological and reproductive ³³⁴ effects of exposure to elevated temperatures, and the limits of physiological adjustment, are documented for very few species.^109,333 Still less is known about the ecosystem-level consequences of repeated exposure to thermal stress and compounding impacts, but range shifts and altered community assemblages are already occurring.^109,335,336

Many coral reef invertebrates have obligate associations with live corals, or rely on particular coral species for food and shelter,^319,337 so are sensitive to coral declines. Calcifying invertebrates, which build carbonate shells or skeletons, such as snails, crabs and sea urchins, are highly sensitive to ocean acidification, which can affect shell growth rates and composition.³³⁸ Elevated carbon dioxide concentrations in sea water also affect foraging and predatory behaviours ^339,340 and reproduction ³⁴¹ in molluscs, potentially via mechanisms that disrupt the nervous system.³⁴² Cross-generational studies suggest some species, including crown-of-thorns starfish (Section 3.6.2), may have the capacity to acclimatise to increasing sea temperatures ^343,344 and elevated carbon dioxide.³⁴⁵ Reductions in water quality affects coastal and inshore areas and associated invertebrate fauna.³⁴⁶

Exploitation of target species can also affect invertebrate populations. The black teatfish fishery reopened in 2019 with stocks at around 40 per cent of their historical unfished biomass following 2 decades of closure,³⁴⁷ while the saucer scallop stock is currently assessed as depleted ³²⁹ (Section 5.4).

Current knowledge of the condition (and trend in condition) of invertebrates in the Region is limited, partly due to the sheer number of species and the variations in life history and habitat requirements across this broad taxonomic grouping. The condition of invertebrates associated with coral reef habitats is affected by the cover, extent and condition of corals. Coral-dependent species are likely to be directly impacted by disturbances that affect corals.^204,348 In contrast, increasing production of fresh coral rubble is likely to benefit the highly diverse invertebrate fauna that occupy its reticulated surfaces and interstitial spaces.^{34,204,349,350} As rubble beds erode, however, their three-dimensional structure becomes less complex, providing habitat for fewer species.³⁴ For non-reef invertebrates, it may be possible to draw broad inferences from the condition and status of other habitats within the Region, such as Halimeda bioherms,²²⁵ seagrass beds, lagoons and mangrove forests. There is limited knowledge on how wider pressures are affecting the condition of non-coral marine invertebrates.

Significant variation exists in condition (and trend in condition) of different species within this diverse group. Pressures associated with climate change, habitat loss, fishing and poor water quality are likely to have affected invertebrates across a range of habitats within the Region.