Belonging to the phylum Echinodermata, the crown-of-thorns starfish, Acanthaster planci, is a radial marine animal. It is related to sea urchins, sea lilies and sea cucumbers, and is very poisonous. It is found in tropical shallow waters in association with coral reefs. It reaches a diameter of 60 cm and may live for eight years. It feeds on coral polyps. In the late 1960s the crown-of-thorns appeared in increasing numbers on the Great Barrier Reef in north-eastern Australia. By 1968 the area from Gladstone to Cooktown in the north had been attacked, and by the next year more than 250 km2 of reef were dead and the tropical fish had disappeared.

Scientists from many nations collaborated in research to determine the cause of the infestation. Among other things, theories advanced have suggested that humans had been the catalyst, either by polluting the water and killing the organisms which had once fed on starfish larvae or by overfishing the marine species which had preyed on adult starfish, such as the triton trumpet shell, trigger fish, puffer fish and the painted shrimp; that warming of the water had encouraged the population upsurge; and that it was a natural phenomenon occurring at irregular intervals. Examination of material from the ocean bed supported this last theory.

The crown-of-thorns population did not maintain its high level and by the 1970s the Commonwealth and Queensland governments concluded that the starfish did not constitute a threat to the existence of the Great Barrier Reef. Regeneration of damaged coral is possible but in many areas the reef-building stony corals have been replaced by soft corals.