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Crown of thorns

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Crown of thorns starfish / Acanthaster planci / Family Acanthasteridae
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Divers with large Crown of Thorns
One of the most commonly known sea stars on the Great Barrier Reef is the crown of thorns starfish. This is one of a few animals that feed on living coral tissue. It gets its name from the dense covering of long sharp spines covering its upper surface. In small quantities this animal is just another part of the coral reef ecosystem, however, when the crown of thorns starfish occur in great numbers they can eat corals faster than corals can grow and reproduce, leading to major reductions in coral cover.

The crown of thorns starfish is a coral-eating species found throughout the Indian and Pacific Oceans. It belongs to the echinoderms and plays a major role in the destruction of fast growing coral species. Scientists believe that this starfish has lived on the Reef for tens of thousands of years.

Crown of thorns starfish physiology

Growing up to 80 centimetres in diameter and covered in numerous sharp, 40-50mm spines, this large starfish makes a formidable sight when encountered under water. Most sea stars have five arms but the crown of thorns has up to 21.

The local crown of thorns starfish is a dull greyish green with pale tinges of red. These colours help the young starfish to blend in and hide.

Some overseas crown of thorns starfish are more brightly coloured. South-east Asian varieties can be a bright blue or purple.

Habitat

The Great Barrier Reef is home to the crown of thorn starfish. The Reef gives the starfish shelter and a place to hide when it is young and vulnerable. More importantly, the reef is the major source of the adult starfish's main diet, coral.

There are over 400 species of hard corals living on the Great Barrier Reef and each reef is well covered with these animals. With such a huge supply of this potential food the reef provides the perfect environment for crown of thorns starfish. It is only one of many animals that live on the reef by feeding on coral.

Feeding

The crown of thorns starfish prefers to eat the faster growing and more common species of corals. These include the branching staghorn corals and the large, flat, plate corals. In choosing to eat these fast growing corals the crown of thorns starfish may help the slower growing corals to compete for space on the reef. When hard coral runs out, foods such as soft corals, algae, clams, encrusting organism's gastropods, sea anemones gorgonians and hydrozoans become the diet.

Reproduction

The term used for reproduction of the crown of thorns starfish is spawning.

A single female crown of thorns starfish can produce up to 100 million eggs per year. Under the water, crown of thorn starfish stand on tiptoes on the tops of rocks. Early summer is the main spawning time, and can happen at any time, day or night, as long as the water temperature is right. From pores, sperm and eggs stream out into the surrounding water where they meet and fertilise.

Crab protects corals against crown of thorns starfish

A tiny crab called Trapezia cymodoce protects the coral Pocillopora damicornis from being preyed upon by crown of thorns sea stars by breaking their spines off.