The crown-of-thorns starfish has relatively few natural predators, largely because its body is covered in venomous spines and laced with toxic compounds called saponins. But several species have evolved ways to get past these defenses, and their presence on a reef turns out to be one of the most important factors in preventing the massive starfish outbreaks that destroy coral.
The Giant Triton Snail
The giant triton snail is the most famous and effective natural predator of adult crown-of-thorns starfish. This large sea snail, which can grow over a foot long, has a remarkable attack strategy. It pins the starfish down with its muscular foot, then extends a proboscis (up to 400 mm long) to pierce the starfish’s outer skin near its central disc. It injects a paralyzing cocktail of venom and sulfuric acid from specialized salivary glands, immobilizing the starfish within moments.
Once the starfish is paralyzed, the triton wraps it in thick mucus and uses a rasping tongue-like structure to shred through the spiny outer skin. The entire process, from hunt to complete consumption, takes anywhere from 4 to 24 hours. In one study, 15 triton snails housed with 100 adult starfish consumed an average of 1.5 starfish per week each. In another, just two tritons ate ten small and three large starfish per month.
What makes the triton snail truly unusual is its immunity to the starfish’s toxic saponins. Special enzymes in its liver break down these compounds by stripping away the part of the molecule that makes them dangerous, converting them into harmless sterols. Unfortunately, giant tritons have been heavily collected for their ornamental shells, and their declining numbers on Indo-Pacific reefs may be one factor contributing to starfish outbreaks.
Reef Fish That Eat Starfish
Several large reef fish prey on crown-of-thorns starfish at various life stages. The humphead wrasse, one of the largest fish on coral reefs, actively feeds on crown-of-thorns starfish along with other toxic prey like sea hares and boxfish. Its powerful jaws can crush through the spiny exterior that deters most other fish.
Research published in Nature Communications found that the density of crown-of-thorns starfish increases systematically as fishing removes more fish biomass from a reef. The relationship is striking: reefs open to fishing have nearly three times more individual starfish compared to reefs within no-take marine reserves. The fish families most strongly linked to starfish control include emperors, tropical snappers, and groupers. Even small amounts of fish biomass removal correlated with starfish populations crossing the threshold toward outbreak levels.
Butterflyfish also play a role, particularly in consuming starfish larvae. Lab studies have shown that butterflyfish appear insensitive to the toxic chemicals in starfish larvae, eating them at consistent rates regardless of developmental stage. The total number of fish species reported to potentially prey on starfish larvae has reached 84.
Harlequin Shrimp
Harlequin shrimp are tiny, vividly colored predators that feed almost exclusively on starfish, including crown-of-thorns. They hunt in monogamous pairs, working together to flip a starfish onto its back and drag it to a sheltered spot. The process can take considerable effort, with the starfish repeatedly trying to right itself while the shrimp persistently flip it back over. Once secured, the pair feeds on the starfish over several days, consuming it from the tube feet inward. Their small size means they have more impact on juvenile starfish than full-grown adults, but their specialist diet makes them a consistent source of predation pressure.
What Eats the Larvae
Crown-of-thorns starfish are most vulnerable during their larval stage, when they drift as plankton before settling on the reef. Corals themselves are among the most significant predators at this stage. Filter-feeding coral species like cauliflower coral capture and consume settling starfish larvae alongside other plankton. Anemones also feed on them. Because coral reefs support enormous numbers of planktivorous fish and filter-feeding invertebrates, the sheer volume of mouths on a healthy reef can exact a heavy toll on starfish larvae during settlement.
That said, the starfish have evolved a counter-defense. Their eggs and larvae contain saponins at concentrations high enough to deter many planktivorous fish. In experiments, damselfish discriminated against food particles containing saponin extract even at concentrations 10,000 times lower than what’s found in actual starfish eggs and larvae. Hungrier fish were less picky, but the chemical defense still significantly reduced predation. This is one reason why, despite the huge number of potential larval predators, enough larvae survive to occasionally fuel explosive population growth.
Why Predator Loss Fuels Outbreaks
Crown-of-thorns outbreaks are responsible for roughly 40% of the coral loss recorded on the Great Barrier Reef between 1985 and 2012, corresponding to a decline of about 1.4% coral cover per year. The link between predator removal and these outbreaks is now well supported. Reefs where fishing has reduced predator populations see starfish densities climb to outbreak levels, while unfished reefs maintain markedly lower starfish numbers. The biomass of fish species known to influence starfish densities is 1.4 to 2.1 times higher inside no-take marine reserves compared to fished reefs.
Where natural predators alone cannot keep up, human intervention has stepped in. On the Great Barrier Reef, professional divers inject starfish with a solution that kills them on contact. The results have been significant: in sectors that received timely and sufficient culling effort during the most recent outbreak wave, coral cover increased by 44%, while comparable sectors with little or no culling lost 37% of their coral. Reefs with the highest levels of culling effort saw net increases in coral cover, and the progression of outbreaks to neighboring reef sectors was delayed, likely because killing adult starfish reduced the supply of larvae drifting downstream.
Still, these manual control programs work best at local scales. The long-term solution, researchers increasingly argue, involves protecting the natural predators that kept starfish populations in check before overfishing disrupted the balance. Maintaining healthy populations of large reef fish and giant triton snails may be one of the most practical tools available for preventing the next outbreak wave.