Marine Animal Encyclopedia
Crown-of-thorns Starfish Acanthaster planci
The Crown-of-thorns Starfish is a sea star named for the spines that cover its body and arms. These spines are somewhat flexible in life and are used for locomotion and for defense from potential predators. This species is a well-known coral predator, and outbreaks of tens of thousands of individuals have been known to cause serious harm to coral reefs in some areas. These outbreaks may be a result of overfishing of the Crown-of-thorns Starfish’s primary predator, the Giant Triton or they may be a natural phenomenon.
These starfish are known to be more successful at preying on large swaths of coral reefs when the corals are already stressed. During times of coral bleaching or stresses caused by human activities, outbreaks of the Crown-of-thorn Starfish may be particularly destructive. These starfish feed by inverting their entire stomach, through the mouth, and digesting the thin layer of soft tissue off of a coral’s skeleton, right in the open environment, and sucking down the available nutrients.
Historically, fishers and conservationists who feared that Crown-of-thorns Starfish would permanently damage local reefs have worked together to try to control the population size of this predator. In some places, individuals were chopped in half or into smaller pieces. Unfortunately, this process may have backfired, as Crown-of-thorns Starfish can regenerate arms and in extreme cases may be able to regenerate from only half of an animal. Current efforts to control Crown-of-thorns Starfish include complete removal from the reef or poisoning with substances that kill the starfish but not other species on the reef (particularly corals).
At least one group of animals associated with specific corals is known to attack the Crown-of-thorns Starfish in order to protect their homes. The guard crabs (genus Trapezia) live amongst the branches of cauliflower corals and other branching corals and are known to defend their home colonies from Crown-of-thorns Starfish that are trying to feed on them. The crabs pinch the starfish’s tube feet or even its stomach lining. Through this symbiosis, the crabs protect the coral colony from potential predators and in return, they receive a safe place to live and to avoid their own predators.
These starfish reproduce through a behavior known as broadcast spawning, where several females release eggs and several males release sperm into the water column above the reef, all at the same time. This method increases the likelihood that eggs will become successfully fertilized and that fertilized eggs will not be eaten by egg predators on the reef surface.
Crown-of-thorns Starfish populations fluctuate between outbreaks with very high densities and times with much fewer individuals. People do not utilize this species, but scientists consider it a species of concern – not because its numbers are too low but because locally high numbers can threaten other species.