Baby crown-of-thorns starfish don't look particularly threatening.
In their first stage of life, the starfish measure less than a millimetre across, and by the time they reach their first year, the creatures are the size of your fingernail.
But by adulthood, they reach dinner-plate proportions and have an appetite to match.
The starfish's voracious hunger for coral has seen its presence extend from the northern Great Barrier Reef down to the Capricorn Cays, off the central Queensland coast, over the past decade.
"I think of them as locusts going through a farmer's crop," the University of Queensland's Kenny Wolfe said.
Dr Wolfe and his small team of researchers are at the Heron Island Research Centre, in the Southern Great Barrier Reef, searching for natural ways to halt the starfish in its tracks.
"When they are delicious little jellybeans, we're hoping that we can find an assortment of predators from the reef that can consume the crown-of-thorns in their early baby stages," Dr Wolfe said.
Over two months the team has tested more than 100 species of invertebrates, including crabs, worms, and snails that they found in coral rubble.
The piles of dead coral skeletons are a perfect hiding place for juvenile starfish.
The team has found 27 species that eat the crown-of-thorns and it is the decorator crab Schizophrys aspera that leads the pack.
"They decorate themselves with pretty-coloured sponges and things, which are toxic and help them camouflage in the environment," Dr Wolfe said.
He said very little was known about these crabs and snails because they were often overlooked, but they could be important soldiers in the war on the starfish.
"That might link to the reefs that do or don't have outbreaks already of the starfish, and if we can start just understanding these species on the reef, that can filter more through to management in the future," Dr Wolfe said.
A delicate balancing act
The Great Barrier Reef Marine Park Authority (GBRMPA) is responsible for managing the reef and when it comes to the crown of thorns, it's about suppressing them to a manageable level.
David Williamson heads up GBRMPA's crown-of-thorns control program, which gets to about 200 of the marine park's 3,500 reefs a year.
"Some reefs seem to be free of outbreaks for one reason or another, but some reefs tend to get hit time and time again," Dr Williamson said.
The challenge is identifying which reefs to prioritise and, according to Dr Williamson, it comes down to protecting key sources of coral.
"Corals that produce babies, which are dispersed on the water columns, are the ones that go and reseed the surrounding reefs and actually replenish the coral populations," he said.
Crown-of-thorns starfish are a natural part of the ecosystem — just not in these numbers.
"We've changed this ecosystem," Dr Williamson said.
"We've been clearing the land for 150 years or more, we grow sugar, we graze cattle, we build up urban areas, we put our waste into the sea."
Finding natural predators
On top of that, humans have removed a fair number of their predators, particularly some fish species that target the starfish when it's very small.
"What we've seen from some recent research, published by the Australian Institute of Marine Science, is that starfish outbreaks are three times more common on reefs that are open to fishing," Dr Williamson said.
He has welcomed this latest research from Heron Island.
"We weren't really aware that fish were big predators of starfish, and now this new study is showing us that there's a whole lot of other stuff, including these crabs, that are potentially really important predators as well, particularly when the starfish are juveniles," Dr Williamson said.
He said the answer might lie in protecting the predators that go for baby crown-of-thorns because "they eat a lot in a short period of time".
Very few – if any – creatures prey on adult starfish.
But researchers are trialling one such predator, the giant triton snail, which was almost hunted to extinction, and are looking at ways to reintroduce it to the reef.
Dr Williamson said it could only eat one starfish a week and while it reproduced easily in captivity, getting its larvae to take at the bottom of a reef was a bit more difficult.
GBRMPA's control program has been running for a decade and has just had its funding more than doubled to $162 million up to 2030.
Dr Williamson said the unprecedented investment in the control program was needed to combat the increasing threats to the marine park's resilience.
He said climate change was the number one threat.
"But we can't prevent cyclones and we can't prevent broad-scale coral bleaching events," Dr Williamson said.
While outbreaks cannot be prevented, scientists believe they can be suppressed to a manageable level.
"By a manageable level, I mean a point at which the numbers are low enough so that coral growth and recovery and the reproduction and everything that corals do can actually outpace the rate at which the coral is being eaten by the starfish," Dr Williamson said.
