Crown-of-thorns starfish (Acanthaster planci)

Description and distinguishing features

Crown-of-thorns typically have numerous long, sharp spines arranged on a central disc and on their radiating arms. These spines are rigid and can deliver a painful sting; bright or contrasting colours often serve as a warning to potential predators. For references on appearance and anatomy see spines and skin and general size descriptions such as diameter measurements. Their geographic distribution is widely recorded in regional faunal surveys: Pacific, Indian Ocean, and Red Sea.

Feeding behaviour and ecological impact

This species feeds almost exclusively on reef-building corals. Using its tube feet and arms it climbs onto coral colonies, everts its stomach onto the surface and secretes digestive enzymes to liquefy the living coral tissue before ingesting it. The consumed coral polyp tissue and the exposed calcium-carbonate skeleton are characteristic signs of its feeding; see more on coral relations at coral polyps and polyp biology. Feeding usually occurs at night, and a single starfish can consume large areas of coral over weeks to months.

Life cycle and population dynamics

Crown-of-thorns reproduce by broadcast spawning: adults release eggs and sperm into the water column where fertilization occurs. The resulting planktonic larvae spend days to weeks in the plankton before settling and metamorphosing into juveniles. Under certain environmental conditions — such as elevated nutrient levels and favourable currents — larval survival can be high, producing outbreaks in which many individuals appear on a reef. Outbreaks are a primary cause of rapid coral decline in affected areas.

Management, predators and human interactions

Natural predators include some reef fishes, a few species of shrimp and crabs, and historically the giant triton snail, though predation pressure is often insufficient to prevent outbreaks. Where outbreaks threaten high-value reefs, active control measures are used: manual removal, targeted injection with substances that kill the starfish (for example, bile-salts or other agents), and coordinated culling programs. For summaries of control techniques and monitoring efforts consult digestion and control methods and regional management pages at reef conservation sources.

Notable facts and distinctions

• Common name derives from the crown-like ring of long spines surrounding the central disc.
• Its coral-eating habit makes it an important, sometimes destructive, ecological agent on coral reefs.
• Colour and spine patterning vary geographically and with age; bright colours often indicate the presence of venomous spines.

Because the crown-of-thorns can both occur naturally at moderate densities and explode into damaging outbreaks, it remains a focus of ecological study and reef management. Research continues into the causes of outbreaks, effective non-harmful control methods, and the species' role in the resilience and recovery of coral reef ecosystems.