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The crown-of-thorns starfish

Crown-of-thorns starfish ( Acanthaster planci ) are spiny-looking creatures that inhabit the Indian and Pacific oceans. They are currently considered one of the main causes of the deterioration of the world’s coral reefs.


Crown-of-thorns are a species of starfish that belong to the family Acanthasteridae . Generally, they are between ten and thirty-five centimeters in diameter, but in some cases they can be twice as large.

They are characterized by having between eight and twenty-one arms completely covered by thorns. These are sharp and measure approximately two centimeters. Because of them, this species is popularly known as the Crown of Thorns . Its arms can span a diameter of fifty centimeters.

These starfish are orange, red, or purple in color, and their spines are often yellow, greenish, or pinkish.

Although there are more than a thousand starfish in this family, the crown-of-thorns is the only poisonous species. Its venom causes puncture wounds and can cause pain, nausea, and vomiting.

Habitat and feeding

Crown-of-thorns are animals native to the Indo-Pacific region. They live in calm and deep waters. It is common to also find them in the Red Sea, in Hawaii, Japan, Australia and other areas of the Indian and Pacific Oceans. In recent years its population has increased rapidly worldwide.

Regarding their diet, crown-of-thorns generally eat the polyps of hard corals, such as staghorn corals. In order to devour its prey, this starfish is placed on top of them and covers them. Then, it takes its stomach out through its mouth and performs external digestion. Through filaments, organic matter enters its interior. Finally, it leaves a white stain on the dead coral.


Reproduction in crown-of-thorns is sexual and occurs by external fertilization. They generally breed during the summer months. Females release eggs and males, sperm, and fertilization occurs in the water. Its reproduction is as peculiar as its appearance: it can lay between ten and twenty-five million eggs.

From fertilized eggs hatch planktonic larvae, which feed on phytoplankton. After completing three growth phases, the larvae settle on a hard substrate and transform into a small five-armed starfish. As it grows, the number of arms also increases.

Threat to other species

The crown-of-thorns starfish is not endangered. However, it sometimes becomes a significant threat to coral reefs.

When crown-of-thorns starfish populations are under control, they can be beneficial to fast-growing corals. But, from time to time, the number of this type of starfish increases excessively. They are considered dangerous when there are thirty or more starfish per marine hectare. At this point, the crown-of-thorns consume the corals more quickly, outpacing their growth rate.

Due to this, it can become a serious threat not only for the reefs but also for the marine fauna that lives or feeds on them.

Other curious facts

  • Crown-of-thorns can lay up to sixty million eggs per season.
  • Despite their great reproduction, they are usually solitary animals.
  • They can move at a speed of twenty meters per hour.
  • The most common predators of crown-of-thorns starfish, particularly of young specimens, are the giant newt snail, stellate pufferfish, and wrasse wrasse.
  • On the French coast of the Mediterranean Sea it is considered an invasive species.


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  • Delgado Cortés, A. Marine zoology, terrestrial vertebrates. (2006). Spain. Azarbe.
  • Morris, D. The world of animals. (2015). Spain. siruela.