Corals are fighting back against reef-killing starfish by stinging their predators with poisonous cells, researchers have discovered.

University of Sydney marine biologist Dione Decker captured juvenile crown of thorns starfish recoiling their legs after being stung by the cells on the arms of stony acropora coral. In some cases the starfish died.

The stingy cells are used by coral to defend themselves and catch food.

Crown of thorns starfish feast on coral and experts says population outbreaks are one of the greatest threats to tropical reef habitats, next to coral bleaching.

Ms Deaker’s supervisor and University of Sydney marine biologist Professor Maria Byrne said the coral stings stunted the starfish arm length by 83 per cent.

“This acropora coral is fighting back against attacking juvenile crown of thorns starfish by using their stinging cells to injure and even kill, showing that coral may not be as passive as people think,” Professor Byrne says.

The marine scientists say that this is the first study of injury and regeneration in juvenile crown of thorn starfish following damage caused by natural enemies.

The researchers isolated 37 young crown of thorns starfish from predators and recorded them being fed acropora coral for more than three months.

Their goal was to discover what was causing tissue damage to young starfish.

“Seeing it caused by coral came as a complete surprise,” Prof Byrne said.

The young juvenile starfish, which have a preference for the fast-growing acropora coral showed that when their tube feet reached out to touch the coral, their entire arm curled back to avoid the corals’ defensive poisonous stinging cells.

Prof Byrne said the sting attacks from the acropora coral delayed the growth of the young starfish, with their arms taking four months to regrow and it also extended the time the fish needed to eat a vegetarian diet.

However the researchers cautioned that although the young starfish’s growth was stunted, they were resilient and mostly recovered.

The findings were published in Marine Ecology Progress Series.

According to the Australian Institute of Marine Science, crown of thorns starfish can kill up to 90 per cent of the corals on affected reefs.

“There are population outbreaks of adult crown of thorns starfish, as a single female can spawn around 200 million larvae which can then clone themselves,” Professor Byrnes says.