Ki‘ai Moku: Too many crown-of-thorns starfish can harm Hawaii reefs

A single crown-of-thorns starfish is impressive. This unusually large, sinister-looking sea star grows to 3 feet in diameter with as many as 19 spiny arms.

It’s hard to miss on the reef, where it munches on coral. Crown-of-thorns have the highest fertilization rate of any invertebrate: each female can produce up to 60 million eggs during a single spawning season.

When too many of these survive, it’s considered an “outbreak.” An overabundance of crown-of-thorns can spell trouble for Pacific reefs. It’s an unusual case of a native animal acting invasively in the marine environment. But it’s not clear that all outbreaks are bad or how environmental changes might exacerbate the problem.

Proliferations of the spiny creatures can destroy 90 percent of a reef, as past outbreaks in Saipan, the Marshall Islands and Guam have shown. In situations where the reef is stressed, an abundance of coral-eating starfish can trigger a cascade of changes. First the corals go, replaced by algal overgrowth. The resulting shift in fish populations can take years to recover.

In Hawaii and Australia, concerns about crown-of-thorns outbreaks have focused on the reduced aesthetic value of the reef, and consequently, a decline in tourism. For some communities, the reef is the icebox, and crown-of-thorns outbreaks can leave it empty.

But outbreaks rarely occur in Hawaii. Many crown-of-thorns starfish larvae die off, while adults are eaten by triton’s trumpet snails, stripebelly pufferfish and harlequin shrimp. A healthy reef can support small numbers of prickly stars, and it’s probable that they benefit the reef in some way.

According to Russell Sparks, aquatic biologist with the Department of Land and Natural Resources Division of Aquatic Resources, “In Hawaii, crown-of-thorns starfish feed on fast-growing, quick-to-settle corals, such as rice and cauliflower coral. These corals can overrun other species like lobe and finger coral, so a periodic bloom of crown-of-thorns could be an important way for reefs to maintain coral diversity.”

In 2004, marine biologists observed a crown-of-thorns outbreak in Ahihi Kinau.

“Although the coral cover impacts were dramatic, the recovery seems to be well on its way,” Sparks said. “It may increase overall coral diversity, which should make the reef more resilient to future disturbances.”

Until recently, scientists hypothesized that crown-of-thorns outbreaks in remote locations such as Hawaii, Guam and French Polynesia resulted from an influx of larvae from elsewhere in the Pacific. In Australia, massive starfish outbreaks spread south along the reef in waves, seeded from larvae upstream.

But new research indicates that Hawaiian blooms occur within the native population. A team of scientists from the University of Hawaii at Manoa looked at the genetics of crown-of-thorns starfish and found that these supposed “invaders” were actually locals – they weren’t some rogue population from across the Pacific. What does this mean?

Crown-of-thorns outbreaks are not fully understood. The species may be acting invasively because of human interference. Some biologists theorize that heavy rainfall and coastal nutrient runoff contribute to a higher-than-normal survival rate for larvae, resulting in a larger number of adults. Over-harvesting of the species’ natural predators could be another potential trigger. Researcher Rob Toonen recommends that marine wildlife managers “seriously consider the role that environmental conditions and local nutrient inputs play in driving crown-of-thorns outbreaks.”

You can help scientists learn more. The citizen-monitoring project Eyes of the Reef relies on reports from regular reef users to monitor reef health.

Crown-of-thorns sea stars are one species of focus. Early detection of outbreaks is critical to protecting the reef. Report any occurrence of 20 or more crown-of-thorns starfish through the Eyes of the Reef monitoring project at

* Lissa Fox Strohecker is the public relations and education specialist for the Maui Invasive Species Committee. She holds a biological sciences degree from Montana State University. Ki’ai Moku, “Guarding the Island,” is prepared by the Maui Invasive Species Committee to provide information on protecting the island from invasive plants and animals that can threaten the island’s environment, economy and quality of life.