Crown-Of-Thorns Starfish Are Gorgeous Killers

Crown-of-thorns starfish (Acanthaster planci) are beautiful, prickly and devastating creatures that have caused mass destruction to some of the world's most beautiful coral reefs.

Description

One of the most noticeable features of the crown-of-thorns starfish is the spines, which may be up to two inches long. These sea stars can be from nine inches to up to three feet in diameter. They have 7 to 23 arms. Crown-of-thorns starfish have a variety of possible color combinations, with skin colors that include brown, gray, green, or purple. Spine colors include red, yellow, blue, and brown. Despite their stiff appearance, crown-of-thorns starfish are surprisingly agile.

Crown-of-Thorns Starfish Facts

• Kingdom: Animalia
• Phylum: Echinodermata
• Subphylum: Asterozoa
• Class: Asteroidea
• Superorder: Valvatacea
• Order: Valvatida
• Family: Acanthasteridae
• Genus: Acanthaster
• Species: Planci

Habitat and Distribution

Crown-of-thorns starfish prefer relatively undisturbed waters, found in lagoons and deep water. It is a tropical species that lives in the Indo-Pacific Region, including the Red Sea, South Pacific, Japan, and Australia. In the U.S., they are found in Hawaii.

Feeding

Crown-of-thorns starfish usually eat the polyps of hard, relatively fast-growing stony corals, such as staghorn corals. If food is scarce, they will eat other coral species. They feed by extruding their stomach out of their bodies and onto the coral reef and then using enzymes to digest the coral polyps. This process can take several hours. After the coral polyps are digested, the sea star moves off, leaving only the white coral skeleton behind.

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Predators of crown-of-thorns starfish (mostly of small/young starfish) include the giant triton snail, humphead Maori wrasse, starry pufferfish, and titan triggerfish.

Reproduction

Reproduction in crown-of-thorns starfish is sexual and occurs through external fertilization. Females and males release eggs and sperm, respectively, which are fertilized in the water column. A female can produce 60 to 65 million eggs during a breeding season. Fertilized eggs hatch into larvae, which are planktonic for two to four weeks before settling to the ocean bottom. These young sea stars feed on coralline algae for several months before switching their diet to corals.

Conservation

The crown-of-thorns starfish has a healthy enough population that there is no need to evaluate it for conservation. In fact, sometimes crown-of-thorns starfish populations can get so high, they devastate reefs.

When crown-of-thorns starfish populations are at healthy levels, they can be good for a reef. They can keep larger, fast-growing stony corals in check, allowing small corals to grow. They also can open space for more slower-growing corals to grow and increase diversity. 

However, about every 17 years, there is an outbreak of crown-of-thorns starfish. An outbreak is said to occur when there are 30 or more starfish per hectare. At this point, the starfish consume coral faster than the coral can regrow. In the 1970s, there was a point when 1,000 starfish per hectare were observed in a section of the northern Great Barrier Reef.

While it appears these outbreaks have happened cyclically for thousands of years, recent outbreaks seem to be more frequent and severe. The exact cause is unknown, but there are some theories. One issue is runoff, which washes chemicals (for example, agricultural pesticides) from the land into the ocean. This pumps more nutrients into the water that causes a bloom in plankton, which in turn provides extra food for crown-of-thorns starfish larvae and causes the population to boom. Another cause may be overfishing, which has decreased the population of starfish predators. An example of this is the overcollection of giant triton shells, which are prized as souvenirs. 

Scientists and resource managers are seeking solutions to crown-of-thorns starfish outbreaks. One technique for coping with the starfish involves poisoning them. Individual starfish must be poisoned manually by divers, which is a time- and labor-intensive process, so it can only feasibly be conducted over small areas of a reef. Another solution is to try to prevent outbreaks from happening or stop them from becoming so large. One way to do that is through working with agriculture to reduce pesticide use, and through practices such as integrated pest management. 

Use Care When Diving

When snorkeling or diving around crown-of-thorns starfish, use care. Their spines are sharp enough to create a puncture wound (even though a wet suit) and they contain a venom that can cause pain, nausea, and vomiting.

Resources and Further Reading

"Acanthaster planci (Linnaeus, 1758)." World Register of Marine Species.

Becker, Joseph. "Marine Envenomations: Invertebrates." Alert Diver Online, Paul Auerbach, Dan Holdings, Inc., Spring 2011.

"Crown-of-thorns starfish." Australian Institute of Marine Science, Australian Government, 2019.

"Crown of Thorns Starfish." Reef Resilience Network, The Nature Conservancy, 2018.

Hoey, Jessica. "Environmental Status: Crown-of-thorns starfish." Great Barrier Reef Marine Park Authority, Australian Government, August 2004.

"Injection culls reef-killing crown of thorns starfish." The Sydney Morning Herald, April 22, 2014. 

Kayal, Mohsen, et al. "Predator Crown-of-Thorns Starfish (Acanthaster planci) Outbreak, Mass Mortality of Corals, and Cascading Effects on Reef Fish and Benthic Communities." PLOS ONE, October 8, 2012.

Shell, Hanna Rose. "Locomotion in Water." Scinema Study Guide, CSIRO.