Crown-of-Thorns Starfish

A Sea Star That is a Voracious Coral Reef Predator

Crown-of-thorns Starfish / Georgette Douwma / Photographers Choice / Getty Images
Crown-of-Thorns Starfish, Komodo, Indonesia. Georgette Douwma / Photographers Choice / Getty Images

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

Description

One of the most noticeable features of the crown-of-thorn starfish is their spines, which may be up to two inches long. These sea stars can be from 9 inches to up to 3 feet in diameter. They have 7-23 arms.  These are colorful animals with a variety of color combinations.

Skin colors include brown, gray, green or purple up to 2 inches long. Spine colors may include red, yellow, blue and brown.  Despite their stiff appearance, crown-of-thorns starfish are surprisingly agile.

Classification

  • 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, such as that found in lagoons and deeper water. It is a tropical species that lives in the Indo-Pacific Region, including in 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, but 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. 

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

Reproduction

Reproduction is sexual, with external fertilization. Females and males release eggs and sperm, respectively, which are fertilized in the water column. A female can produce 60-65 million eggs during a breeding season. Fertilized eggs hatch into larvae, which are planktonic for 2-4 weeks before settling to the ocean bottom. These young sea stars feet on coralline algae for several months before switching their diet to feed on corals.

Conservation

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

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 1970's, according to Reef Resilience, 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 cyclicly 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 (e.g., agricultural pesticides) from the land into the ocean. This pumps more nutrients into the water. This causes a bloom in plankton, which in turn provides extra food for crown-of-thorns starfish larvae, and causes that population to boom.  Another cause may be overfishing, which has decreased the population of predators of the starfish. An example of this is 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 being so large. One way to do that has been through working with agriculture to reduce pesticide use and using practices such as integrated pest management. 

To report crown-of-thorns starfish sightings in Australia or learn how to be part of the eradication program, click here

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 through a wet suit) and they contain a venom that can cause pain, nausea and vomiting.

References and Further Information

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Kennedy, Jennifer. "Crown-of-Thorns Starfish." ThoughtCo, Aug. 29, 2016, thoughtco.com/crown-of-thorns-starfish-2291456. Kennedy, Jennifer. (2016, August 29). Crown-of-Thorns Starfish. Retrieved from https://www.thoughtco.com/crown-of-thorns-starfish-2291456 Kennedy, Jennifer. "Crown-of-Thorns Starfish." ThoughtCo. https://www.thoughtco.com/crown-of-thorns-starfish-2291456 (accessed January 23, 2018).