Crown of Thorns Starfish


The Crown of Thorns Starfish (COTS)

COTS - At a Glance:

The crown-of-thorns starfish (Acanthaster planci), is a large starfish that preys upon hard, or stony, coral polyps. The crown-of-thorns starfish receives its name from venomous thorn-like spines that cover its upper surface, resembling the biblical crown of thorns. It is one of the largest starfish in the world!


A Crown-of-Thorns Starfish at Juans Bay - Raja Ampat

The Crown of Thorns Starfish (often abbreviated to COTs) is a coral-eating predator, and quite dangerous when its population increases. It preys on reef corals, destroying 5 to 13m2 per year (Lassig, 1995).

In ideal conditions, the presence of the Crown of Thorns Starfish’s function is to maintain the balance of the coral reef ecosystem as the Starfish will prey on reef corals whose growth is predominant, such as Acropora. Ideally, the number of Crown of Thorns Starfish present in the ecosystem should not exceed 30 individuals per hectare.

Outbreaks of Crown-of-Thorns Starfish are considered a severe threat to coral reefs. On the Great Barrier Reef in Australia, around 25% of coral has been directly destroyed by the starfish in the past 30 years. In other areas, even higher rates of up to 90% have been recorded.


 White dead coral killed by Crown-of-Thorns

COTS - The Facts:

  • The Crown of Thorns Starfish or Acanthaster planci is one of two sub-species of the genus Acanthaster (Starfish) widely distributed throughout the Indo-Pacific region.
  • It is a type of starfish armed with many venomous spikes, and in Indonesia is better known as Bulu Seribu (Thousand Bristles) (Suharsono, 1991).
  • It has in excess of 21 arms. The upper part of its body is covered in poisonous spikes. An adult COTS diameter can reach a size of one meter and bigger (COREMAP, 2004).
  • Its unique body shape resembles a disk with a large and flat stomach (Birkeland and Lucas 1990).
  • COTS presents in different colors depending on the area where they live. In Indonesia, the COTS are usually grey, purple, green and blue (Moran, 1990).
  • It grows fast. Crown of Thorns Starfish’s growth rate could reach up to 5 cm in the first year, and may reach 30 cm at 2 years of age (Fraser et al.,2003).

COTS - Reasons for increased Numbers:

  1. Natural factors: Increased of both nutrient concentrations and availability of food for the Crown of Thorns Starfish coupled with other factors causing the population’s explosion (exceeding desirable population limit).
  2. Exploitation of natural predators of the Crown of Thorns Starfish: Triton Shell, Red Snapper, Prawns, and Lobsters.
  3. Flow of nutrients from coastal activities: Increase food for the starfish from the river systems to the sea, such as: extensive use of fertilizers, domestic waste.