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Facts About the Crown-of-Thorns Starfish

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Facts about the crown-of-thorns starfish.

The crown-of-thorns starfish, scientifically known as Acanthaster planci, are considered one of the serious problems besetting a coral reef ecosystem. The reason is that the crown-of-thorns starfish feed on corals, removing the polyps and leaving the corals bleached and dead. The feeding frenzy is especially pronounced during outbreaks of the species.

Why are these starfish called crown-of-thorns?

By simple inspection, it is very easy to see that the crown-of-thorns starfish reminds one of the thorny crown forced on Christ's head. The crown-of-thorns starfish are covered with long spines (ca. 5 cm) projecting from its body and 13 to 16 radiating arms. The venomous spines are used mainly for defense against predators including humans. They can grow to as much as half a meter in diameter.

crown-of-thorns starfish

Three crown-of-thorns starfish on a tabulate coral colony (Photo by Ofer Ben-Tzvi)

How do the crown-of-thorns starfish feed?

The crown-of-thorns starfish feed by placing itself above the coral, preferably the tabulate (table-like) corals, and turns upside down (evert) their stomach through their mouth. Once the food is covered with their stomach, the coral polyps are easily digested with enzymes. This way of feeding , render many of the coral reefs destroyed. Each crown-of-thorns starfish can eat up to a one square meter of coral. Outbreaks could last from one to five years or even longer. They feed actively especially at night. Thus, the crown-of-thorns starfish are nocturnal, although they have been observed feeding during the day.

How do the crown-of-thorns starfish reproduce?

The crown-of-thorns starfish can spawn millions of eggs a year but many of these are lost due to predation. They have free-swimming larvae which gets dispersed with the plankton in the oceans. These larvae eventually become juveniles with five arms through metamorphosis where there is a complete change in their form. Adulthood is reached in about two years which repeat the process of reproduction.

What are the predators of the crown-of-thorns starfish?

Some scientists explain that the reason for crown-of-thorns outbreaks is due to the loss of its important natural predators which man collect as food. These predators include triton shells, prawns, and trigger fish. Other researchers attribute the outbreak to high levels of nutrients in the coastal areas, which generally accumulate due to human activities offshore or water runoff.

triton

Triton shells are one of the major predators of crown-of-thorns starfish.

Positive Effects of Crown-of-Thorns Outbreaks?

Although many scientists agree that the crown-of-thorns starfish is destructive to the coral reef ecosystem, a study suggests that in reality the starfish outbreak can even be positive to the coral reef. The outbreak prevents dominance of a coral reef species thus bring about higher biodiversity in the reef ecosystem. Biodiversity is important for coral reef resilience and sustainability. This is no excuse, however, for allowing the crown-of-thorns starfish to proliferate since fishery production is reduced with the loss of the critical coral reef habitat.

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Patrick Regoniel

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