The Great Barrier Reef's most notorious coral killer may actually be helping it

Crown-of-thorns starfish (COTS) are among the most feared creatures on coral reefs. Covered in sharp, toxin-filled spines, these predators are notorious for consuming coral and leaving widespread destruction in their wake.

However, new research from the University of Sydney suggests these infamous reef destroyers may also have a surprisingly beneficial side. When their populations remain under control, crown-of-thorns starfish can help maintain healthy coral communities and support greater biodiversity.

Native to Australia's Great Barrier Reef, these coral-eating starfish have caused enormous damage during population explosions. Since the 1960s, the reef has experienced four major outbreaks, occurring roughly every 15 years. Declining numbers of natural predators, including tritons and several species of reef fish, have made the Great Barrier Reef and other marine ecosystems more vulnerable to these destructive events.

Scientists Discover That Coral Often Survives Starfish Attacks

In a study published in Ecology and Evolution, researchers investigated what happens to coral after crown-of-thorns starfish feed on it. The team examined feeding scars and remaining living tissue at the University of Sydney's One Tree Island Research Station, located on the Great Barrier Reef.

The study sites were within protected areas where healthy fish populations help keep starfish numbers under control. Unlike other parts of the reef that experience devastating outbreaks, these locations maintain relatively low populations of crown-of-thorns starfish.

The results revealed an unexpected pattern. A team led by Dr. Shawna Foo found that living coral tissue survived nearly 75 percent of the observed feeding events.

"At One Tree Reef, COTS appear to consume a relatively consistent amount of coral during each meal and move on, rather than demolishing the entire coral colony," said Dr. Foo, a University of Sydney Horizon Fellow in the School of Life and Environmental Sciences. "This meant live coral tissue remnants were frequently left behind."

How Coral-Eating Starfish Can Boost Reef Biodiversity

The discovery suggests that small populations of crown-of-thorns starfish may play an important role in maintaining the balance of coral reef ecosystems.

Some coral species grow quickly and can take over large areas, limiting the space available to slower-growing varieties. By feeding on these dominant species, crown-of-thorns starfish may create opportunities for other corals to survive and establish themselves.

The researchers found that surviving fragments of fast-growing Acroporidae corals did not regenerate over the areas damaged by starfish feeding. In contrast, regrowth occurred primarily among slower-growing Montipora and Porites corals.

"By feeding on fast-growing corals such as Acropora, low-density COTS populations create space for slower-growing coral species to persist, helping to increase coral diversity," Dr. Foo said.

This finding challenges the idea that all coral consumption by crown-of-thorns starfish is harmful. Under the right conditions, their feeding behavior may actually prevent a few dominant coral species from taking over the reef.

Why Eliminating Crown-of-Thorns Starfish Could Be a Mistake

Despite these potential benefits, researchers caution that large outbreaks of crown-of-thorns starfish remain a serious threat. When their numbers increase dramatically, they can destroy extensive areas of coral, leaving reefs struggling to recover.

The key difference is whether starfish populations remain at naturally manageable levels or grow into destructive outbreaks.

"It's important to distinguish between COTS populations performing their natural role as coral predators and outbreaks causing sustained reef decline."

Although the research was conducted mainly in protected areas with healthy fish populations and little risk of major outbreaks, the findings could help scientists and conservation managers improve strategies for controlling crown-of-thorns starfish.

In particular, the results suggest that removing every starfish may not be the best approach once an outbreak has been brought under control.

"The implications of our work can help us decide when and why culling should occur and the ecological endpoint of a cull. Once an outbreak is suppressed, the goal should not necessarily be to eliminate COTS entirely. Culling needs to be complemented by strong marine management to protect COTS predators and keep COTS populations in check."

Research co-author Professor Maria Byrne connected the findings to a well-established ecological concept.

"The results are consistent with the intermediate disturbance hypothesis, an important framework in ecology and conservation."

According to this hypothesis, ecosystems can benefit from occasional, moderate disturbances that prevent any single species from becoming too dominant.

The research suggests that crown-of-thorns starfish may be an example of this natural balancing process. While uncontrolled outbreaks can devastate coral reefs, smaller populations may help maintain the diversity that makes these ecosystems so valuable.