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The Explorer's Guide contains a treasure trove of aquatic science resources developed for use in K-12 classrooms. Back to Explorer's Guide.

Crown of Thorns Sea Star Scientific Name: Acanthaster planci
Crown of thorns sea stars only eat corals. After covering its prey, this sea star pushes out its stomach through its mouth. Strong stomach acids breakdown the coral polyps, and then small hairs collect the gooey food and carry it inside. Once it is done eating, the sea star pulls the stomach back into its body and leaves behind a white coral skeleton.

Region: Philippines

These are one of the most beautiful sea stars in the world. Sporting bright orange-red and purple skin with stiff yellow or hot pink spikes sticking out, the crown-of-thorns uses these bright colors to warn would-be predators to steer clear. Brightly-colored and irregularly-patterned skin is a sign of warning coloration, colors and patterns that alert other animals it is poisonous or venomous.

Unlike most other sea stars that have five arms, the crown-of-thorns has 12 to 19 arms. This star can grow to 20 inches (50 cm) in diameter.

Found all over coral reefs, these sea stars live in areas crowded with corals, sponges and other invertebrates.

Crown-of-thorns sea stars live in the warm areas of the Pacific Ocean, Indian Ocean and Red Sea.

Famous for its destructive eating habits, crown-of-thorns sea stars feed on living coral polyps. Noshing on this main staple leaves behind white patches of dead coral all over the reef. Crown-of-thorns can be found alone feeding on the reef at night.

Since they don’t have eyes, crown-of-thorns hunt using their sense of smell. Once a delicious smell is detected, the sea star carries itself over to its polyp prey. The mouth of sea stars is on the oral surface or underside of their body. The star covers its prey, then pushes out its stomach from inside its body and covers a colony of polyps. Stomach juices smother the polyps, and cilia or tiny hairs move its gooey meal inside the sea star’s body.

Crown-of-thorns sea stars reproduce by spawning or releasing eggs and sperm into the ocean at the same time. Females can produce up to 65 million eggs per each spawn. Since the egg and sperm are spawned or released into the sea, fertilization or the joining of egg and sperm to form a tiny sea star, is most likely to occur if a large number of sea stars have gathered in the same area and spawn at the same time.

Sea stars go through five growth stages before coming to be the star-shaped animals with which we are familiar. During the first month, the sea star freely floats around looking like a tiny sea jelly or blob. It can barely be seen by the eye and feeds on tiny plants and animals floating in the ocean. The baby sea star then metamorphoses or changes shape into a star-shaped creature. For the next six months, the juvenile sea star slowly grows, finds a hidden home under reef rock and rubble, and begins feeding on algae. After they get big enough, these sea stars emerge and begin to travel the reef in search food. After two years the sea star stops growing and is developed enough to reproduce.

Sea stars can asexually reproduce. This means they can make an exact copy of themselves without eggs and sperm. Sea stars can regenerate or regrow arms if they are bit or ripped off by a predator or grow a new individual from a ripped off arm.

>A species or group of organisms that is in danger of extinction or disappearing from the face of the earth in the near future if its situation is not improved.

A species that can be found throughout its natural range but is declining in number and may become endangered in the absence of special protection efforts.

A species that is not declining in number but is of special concern because it is sensitive to pressure by particular human activities or natural events.

A species that is not declining in number and is not sensitive to pressures by human activities or natural events.

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Threats and Management
Large numbers of crown-of-thorns sea stars can devastate reefs. Found as a natural population with low numbers, these sea stars keep a balance on the reef. They can only eat as much coral as can be replaced by the growth of new coral.

This animal has been the subject of much debate among the scientific community for over 25 years. In many areas of the world there have been blooms or population explosions which wipe-out large stretches of coral, often killing entire reef ecosystems. Scientists take one of two viewpoints on the cause of crown-of-thorns blooms. Some believe they are caused by natural occurrences in the ocean and others believe the blooms are caused by human impacts. One natural explanation for these population explosions is the collection of its natural predator, triton’s trumpet snail, for its beautiful large shell. Pollution and poisoning by chemicals like pesticides and other farming tools is suggested to be one of the human-caused impacts. It is important for scientists to better understand what is causing crown-of-thorns blooms in order to prevent the destruction of coral reefs by this animal.

Did You Know?
While often called a starfish, these animals aren’t actually fish; they’re invertebrates or animals without backbones.

Out of the 1,800 species of sea stars, the crown-of-thorns may be the only sea star that is venomous. Filled with poison, the stout spines will break off and get stuck under the skin causing a sharp burning pain which lessens to a dull pain that takes a minimum of two days to disappear. The wound also produces swelling, numbness and bruising.

Crown-of-thorns sea stars live symbiotically or benefit from living with tiny shrimp. Usually about one or two shrimp bravely live on the surface among the spines of the crown-of-thorns, but there can be as many as 24 individuals. The shrimp receive protection from the sea star and in return they clean the star’s skin freeing it of unwanted bacteria and dead skin.

Although sea stars are invertebrates, they still have a skeleton. Their skeleton isn’t made of bone though. It’s calcareous or made of a calcium-based rocklike substance. The skeleton is made of tons of small ossicles or plates that easily move to give the sea star flexibility.

Sea stars have a unique means of moving. They use an internal plumbing system called the water vascular system. It’s a system of water canals that run throughout the body. Water is sucked into the sea star through the madreporite or a small hole on the aboral surface or top of the animal. The ends of the canals can be found on the animal’s oral surface as tube feet. The tube feet move when there is a change in the water pressure within the canals. There are thousands of tube feet all over the sea star’s oral surface which are used for movement, capturing food and breathing.

Sea stars can’t see. They have an eye spot on the end of each arm. The eye spot can detect changes in light and dark, but can’t make out distinct shapes, colors or details.

Print Publications:

Allen, G. R. & Stene, R. 1996. Indo-Pacific Coral Reef Field Guide. El Cajon, California: Odyssey Publishing Company. 378p. ISBN# 981-00-5687-7.

Allen, G. R. 1997. Tropical Marine Life. North Claredon, Vermont: Periplus Editions Ltd. 64p. ISBN# 962-593-157-0.

Banister, K. & Campbell, A. 1988. Encylopedia of Aquatic Life. New York, New York: Equinox (Oxford) Ltd. 349p. ISBN# 0-8160-1257-101-X.

Colin, P. L. & Arneson, C. 1995. Tropical Pacific Invertebrates. Beverly Hills, California: Coral Reef Press. 296p. ISBN# 0-9645625-0-2.

Coleman, N. 1999. Dangerous Sea Creatures Aquatic Survival Guide. Australia: Neville Coleman’s Underwater Geographic Pty Ltd. 95p. ISBN# 0-947325-24-7.

Coulombe, D. A. 1984. The Seaside Naturalist. A Guide to Study at the Seashore. New York, New York: Prentice Hall Press. 246p. ISBN# 0-13-79710.

Hoover, J. P. 1998. Hawai’i’s Sea Creatures. A Guide to Hawa’i’s Marine Invertebrates. Honolulu, Hawaii: Mutual Publishing. 366p. ISBN# 1-56647-220-2.

Perrine, D. 1997. Mysteries of the Sea. Lincolnwood, Illinois: Publications International, Ltd. 312p. ISBN# 0-7853-2430-5.

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Vocabulary Words

Aboral surface – The topside of the body.

Asexual Reproduction – Reproduction that does not involve the union of egg and sperm; results in an exact copy of the original animal.

Bloom – A growth in population of several hundred or even thousands of individuals which persists for several months or years, and results in the death of large stretches of coral.

Calcareous – Containing calcium carbonate; a hard calcium-based substance.

Cilia – Tiny hairs that are used for movement.

Coral polyp – An animal that does not move and is attached to a hard substrate.

Ecosystem – All the organisms in living in a certain area and their physical environment.

Extinction – An organism that has not been present on the face of the earth for over 50 years.

Eye spot – A light sensitive pigment that can distinguish between light and dark, but cannot determine distinct colors, shapes or details.

Fertilization – The uniting of egg and sperm to form a new individual.

Invertebrate – An animal without a backbone.

Juvenile – A young version of an adult.

Madreporite – A small hole on the aboral surface that draws water into the water vascular system.

Metamorphosis – A change in the body form of an animal when changing from egg to adult.

Oral surface – The underside of the body.

Organism – A living thing.

Ossicles – Plates.

Predator – An animal that kills another for food.

Prey – An animal that is killed for food.

Regenerate – The development of a missing part or the formation of a complete individual from a broken off part.

Spawn – The release of sex cells, eggs or sperm, into the water.

Species – A group of organisms capable of breeding and producing fertile offspring; organisms that share the same gene pool.

Symbiotic – A relationship between two animals in which one or both benefit.

Tube feet – Clear fleshy extensions of the water vascular system that have a sucker at the end.

Venomous – Having the ability to release poison by means of injection; through spines or teeth.

Warning Coloration – Bright colors or wild patterns warning other animals of the colorful animal’s poisonous or venomous nature.

Water Vascular System – An internal plumbing system of connected water canals that run throughout the body. The ends of the canals can be found on the animal’s surface as tube feet. This system makes moving, eating and breathing possible.

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Crown of Thorns Sea Star
Crown of Thorns Sea Star

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