Environment
Crown-of-thorns sea stars are carnivorous predators that feast on corals and are hard to keep in check—but conservationists are fighting back.
ByMelissa Hobson
Published November 21, 2023
• 6 min read
First, a diver stabs a needle at the end of a long pole into the center of the alien-like creature. Then, a vinegar solution is injected in several spots before the animal is gently pulled away from the coral where it was feeding.
Within two days, the solution will take effect and the predator will be reduced to a pile of goo.
These injections are currently the only way to cull coral-eating sea stars called crown-of-thorns, or COTS.
Native to the Great Barrier Reef and reefs across the Indo-Pacific, crown-of-thorns are ferocious coral predators. They have up to 21 arms, all covered in venomous spines over an inch long, and they ooze a toxic slime when threatened.
When the ecosystem is in balance, COTS don’t cause significant harm. But catastrophic outbreaks can occur that devastate reefs. In one outbreak in Japan, divers had to remove 1.5 million of the predators by hand.
Currently, eDNA samples indicate that a crown-of-thorns outbreak is looming in the northern Great Barrier Reef, but control boats are already at capacity dealing with existing outbreaks. “Somebody should go there, but it would require additional resources for management, which they’re currently struggling to get,” says Sven Uthicke, an ecologist at the Australian Institute of Marine Science.
And while climate change is the biggest threat to reefs—putting about 99 percent of global reefs in danger over the next 20 years—new strategies to keep these predators at bay could help give coral reefs a fighting chance at survival.
Unearthly coral predators
Despite growing over two feet and coming in bright blues, purples, reds, and pinks, these bizarre creatures can be hard to find, often lurking in nooks and crannies.
COTS have a natural boom and bust lifecycle that can cause chaos. In small numbers, they aren’t problematic, but during an outbreak these reef-devouring predators can eat 90 percent of a reef’s live corals. Dead corals provide the perfect habitat for the next generation: juveniles live in reef rubble and eat the algae growing there.
“You go to these reefs after an outbreak, and it’s a graveyard. Almost everything is dead,” and it takes five to 10 years for a reef to fully recover, says Mary Bonin, director of the COTS Control Innovation Program at the Great Barrier Reef Foundation.
These sea stars can reproduce at a remarkable rate. One female can produce over 60,000 eggs during a spawning season. Maria Byrne, a marine biologist at the University of Sydney recalls 10 COTS in her tanks producing around 1,000 juveniles: “It was a mess.”
The Great Barrier Reef is particularly impacted by outbreaks, which spread like wildfire across its interconnected system of over 3,000 reefs spanning an area the size of Italy.
These population explosions may be due to an increase in pollutants, such as chemical fertilizer washing out to sea and creating more phytoplankton—tiny algae—in the water. “COTS larvae eat algae, so the more algae we’ve got, the better the COTS larvae do,” says Uthicke.
Overfishing may have also reduced natural predators that usually control COTS populations. Many species that could keep COTS in check are targeted by commercial and recreational fishing. Areas protected from fishing—known as Green Zones—experience fewer outbreaks.
Why are they so hard to control?
Adult COTS can regenerate themselves in a matter of weeks, so you can’t kill them by cutting them up. When researchers cut one of the starfish into two equal halves, both halves healed themselves almost completely in seven weeks.
Instead, divers manually inject them with vinegar or bile salts, which kill the animals without adding chemicals that might harm other marine creatures into the environment. The solution kills the sea stars within 48 hours, and other marine life then feed on the carcasses.
Larvae are resilient, too. “One of the secrets of the success of crown-of-thorns larvae is that they’re so tough,” says Byrne. They can clone themselves and withstand waters warm enough to kill corals, posing a threat to any corals recovering after marine heatwaves.
Juveniles lie in wait—sometimes for years—for a decline in the adult population, when there is less competition, before they mature and start eating corals. Byrne calls these juveniles waiting to grow up her “Peter Pans”. Culling adults may give them “the opportunity to transition to become a coral predator,” she says.
Hope for the reef
Improved survey methods are helping to give earlier warning of outbreaks. Manta tows—when a snorkeller is pulled along by a boat, counting sea stars as they go—are now using imaging technology in place of humans.
To discern whether hidden COTS are present in the water, scientists use a small dipstick device that detects traces of their DNA, not unlike at-home tests use to detect pregnancy, high blood sugar, or COVID-19. And researchers are exploring the use of chemicals to lure the sea stars to a trap or as deterrents to scare them away—they flee at just a whiff of Triton snails.
Even if these attempts to curtail outbreaks are successful, the region’s reefs face a bigger threat.
“Coral reefs are on the edge because of climate change,” says Byrne. “People are pointing to COTS, but the major cause of coral mortality is temperature, and temperature hits the whole reef.”
Bonin agrees: “The number one priority for the reef is reducing greenhouse gas emissions,” she says. For her, managing crown-of-thorns is “not about killing things,” but a way of protecting the entire ecosystem from climate stressors.
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