What is the Great Barrier Reef's biodiversity?

The Great Barrier Reef's biodiversity is one of the richest marine ecosystems in the world. The reef supports more than 1,500 species of fish, 411 species of hard coral, 30 species of whales and dolphins, six of the world's seven sea turtle species, more than 4,000 species of mollusc, and an estimated 215 species of bird that breed or roost on the reef's islands.

The reef itself is built from coral, a colonial animal consisting of millions of tiny polyps that secrete calcium carbonate skeletons. Hard corals on the reef include branching staghorn corals, mound-shaped brain corals, plate corals, and table corals. Soft corals, sea fans, and anemones add further complexity. The reef is also home to iconic species including the giant clam (Tridacna gigas), the green sea turtle, the dugong (Dugong dugon, a marine mammal that grazes on seagrass), the manta ray, and the white-tip and grey reef sharks.

Coral spawning is one of the reef's most spectacular phenomena. Most species spawn synchronously over a few nights in late spring, releasing bundles of eggs and sperm that drift on the currents. The annual spawn was first documented by reef scientists in 1981 and is now studied across the entire 2,300-kilometre length of the reef. The spawn is timed to lunar and water-temperature cues that coral colonies have evolved over hundreds of millions of years.

Climate change has put the reef's biodiversity under sustained pressure. Mass coral bleaching events in 1998, 2002, 2016, 2017, 2020, 2022, and 2024, caused by ocean temperatures exceeding the upper tolerance of reef-building corals, have killed about half of the shallow-water corals on the northern third of the reef since 1995. Crown-of-thorns starfish outbreaks, agricultural runoff from sugarcane and cattle properties, and tropical cyclones add further stress. Recovery between bleaching events has been incomplete, and the reef's ecological future depends heavily on global emissions trajectories over the next two decades.