Crown-of-thorns starfish

The crown-of-thorns starfish resides in the western Pacific Ocean, most notably in the Great Barrier reef off of Australia. It is named for the many spikes, or thorns, on its back and its basic star-like body shape. Unlike many starfish, which roam along the ocean floor in search of prey, the crown-of-thorns starfish lives on the coral reefs on which it feeds. Outbreaks of large populations of crown-of-thorns starfish have been considered a threat to reef ecology, and linked to human environmental impacts.

Kingdom: Animalia

Phylum: Echinodermata

Class: Asteroidea

Order: Valvatida

Family: Acanthasteridae

Genus: Acanthaster

Species: Planci

As its name suggests, the crown-of-thorns starfish is rounded and star-shaped with many spikes, or thorns, on its body. The crown-of-thorns starfish generally grows to be about sixteen inches (forty centimeters) across, but may grow to be up to two feet (sixty centimeters) across and have as many as twenty-three arms. Unlike the European starfish, which has a tiny body and five arms, the crown-of-thorns starfish has a large round middle with many arms extending from its middle. The middle of the crown-of-thorns starfish holds all of this creature's internal organs. Like other starfish, the crown-of-thorns starfish has many tiny, hollow, tube-like structures on the bottoms of its arms. These tube-like structures are called tube feet. The crown-of-thorns starfish uses its tube feet to help it move along the ocean floor and grasp the corals on which it feeds.

The crown-of-thorns starfish is typically found in the tropical areas of the western Pacific Ocean and the Indian Ocean. Living and feeding on coral, this echinoderm, or spiny-skinned, creature lives mostly on and near coral reefs. The largest number of crown-of-thorns starfish may be found on the Great Barrier Reef of eastern Australia. Typically the crown-of-thorns starfish hides during the day in the darker caverns of the reef and emerges at night to feed on coral.

The crown-of-thorns starfish feeds on the hard polyps, or individual corals, which make up coral colonies. It feeds by climbing onto the coral colony and pressing its stomach out through its mouth on the bottom of its body. The stomach then releases digestive juices onto the coral. These juices soften the coral's hard polyps. The crown-of-thorns starfish continues this process over many hours, changing the hard coral into a soft, soupy liquid which can be swallowed. A single crown-of-thorns starfish can eat up to eight inches (twenty centimeters) of coral each night and may kill up to fifty feet (fifteen meters) of coral in a year. An entire coral reef could be destroyed by starfish over time.

The crown-of-thorns starfish is preyed upon by certain fish, snails, and crustaceans. The most threatening predator of the crown-of-thorns starfish is the giant triton. This flesh-eating sea snail slits open the starfish's body and sucks out its soft tissue. The bearded triggerfish and the grathopyllid shrimp also consume the crown-of-thorns starfish. The triggerfish flips the crown-of-thorns starfish onto its back and bites its soft underside and center. Many other animals avoid the crown-of-thorns starfish for the many spikes on its back. The crown-of-thorn starfish also emits a type of slime that is toxic to predators.

Mating for the crown-of-thorns starfish takes place in midsummer. The male and female crown-of-thorns starfish release millions of sperm and eggs into the water during the mating period. The eggs are fertilized by the sperm in the water and develop into tiny crown-of-thorns starfish larvae. The larvae drift in the water for several weeks and then settle to the sea floor and continue developing into their adult forms. Most crown-of-thorns starfish larvae do not survive their larval stage. The lifespan of the crown-of-thorns starfish is unknown.

Crown-of-thorns starfish are a natural feature of Indo-Pacific coral reefs. However, they can threaten the overall health of a reef by destroying the living coral when they exist in large numbers. Ecologists consider a population of fifteen or more of the starfish in one hectare to be an outbreak. It is unclear what causes these population booms, but scientists believe that multiple factors are likely involved in creating conditions favorable to an outbreak. Notably, many researchers suggest that human environmental impacts may contribute, including nutrient runoff and overfishing of predator species.

Much media attention has surrounded the issue of crown-of-thorns starfish outbreaks on the Great Barrier Reef, due to the reef's important ecological role and value to Australian culture and tourism. The Great Barrier Reef Marine Park Authority noted major outbreaks in the 1960s, late 1970s, early 1990s, and 2010s. It created the Crown-of-Thorns Starfish Control Program in 2012, aimed at reducing populations to protect against damage to the reef; other reef stewardship groups have taken similar action. One culling strategy involves divers injecting vinegar or bile salt into specimens, which kills the starfish without damaging the rest of the environment. Divers must be specially trained for these operations, in part due to the toxins in the starfish's spines that cause pain, swelling, and nausea through a puncture wound.

The other species in the starfish genus Acanthaster are:

• South Indian crown-of-thorns starfish (Acanthaster mauritiensis).
• Red Sea crown-of-thorns starfish (not yet named)
• Great Barrier Reef crown-of-thorns starfish (Acanthaster solaris)

Bibliography

Ault, Larissa, et al. “Acanthaster planci Crown-of-Thorns Starfish.” Animal Diversity Web, 2011, animaldiversity.org/accounts/Acanthaster‗planci/. Accessed 9 Apr. 2024.

“Crown-of-Thorns Starfish.” Australian Institute of Marine Science, www.aims.gov.au/research-topics/marine-life/crown-thorns-starfish. Accessed 9 Apr. 2024.

“Crown-of-Thorns Starfish.” Animalia, 2024, animalia.bio/crown-of-thorns-starfish. Accessed 9 Apr. 2024.