AT A GLANCE
- Common Name: Crown-of-thorns Starfish
- Scientific Name: Acanthaster planci
- Category: Invertebrates
- Subcategory: Echinoderms
- Size: Diameter: 10–14 in (25–35 cm); Exceptional giant specimens reach 28–32 in (70–80 cm); Number of arms: 14 to 21
- Weight: Adults: 2.0–6.0 lb (0.9–2.7 kg); Giant specimens can exceed 8 lb (3.6 kg)
- Lifespan: 8–10 years in wild coral reef environments
- Diet: Carnivore / Specialized corallivore (preys almost exclusively on scleractinian hard stony corals, especially Acropora staghorn and Montipora plate corals)
- Habitat: Tropical shallow-water hard coral reefs and barrier reef platforms
- Range: Indo-Pacific tropical coral reefs from East Africa to Central America
- Conservation Status: Not Evaluated (IUCN Red List) / Highly destructive episodic population outbreaks
Overview
The Crown-of-thorns Starfish (Acanthaster planci), widely known across marine biology as COTS, is a massive, multi-armed, and heavily venomous marine sea star belonging to the family Acanthasteridae. Native to the tropical coral reefs of the Indo-Pacific—including Australia’s Great Barrier Reef—this formidable echinoderm is named for its sinister, armor-like array of hundreds of sharp, needle-like venomous spines covering its upper surface, resembling a thorny crown. Capable of growing over two feet across with up to 21 radiating arms, it is one of the largest and most ecologically infamous marine invertebrates on Earth.
In Indo-Pacific coral reef ecosystems, the crown-of-thorns starfish plays a controversial, high-impact ecological role. At natural low baseline densities, it functions as a healthy disturbance agent that promotes coral biodiversity by preferentially feeding upon fast-growing staghorn and plate corals (Acropora), preventing them from monopolizing the reef and allowing slower-growing brain and boulder corals to establish. However, when agricultural fertilizer runoff and overfishing of natural predators trigger explosive population outbreaks, millions of starfish swarm across reefs, devouring living coral tissue and leaving behind vast, bleached skeletal graveyards.
Biology and Physical Characteristics
Acanthaster planci possesses sensational morphological, biochemical, and regenerative adaptations. Its entire dorsal surface is studded with 2-inch venomous spines coated in toxic saponins and plancitoxins, which cause agonizing, burning pain, localized necrosis, and nausea in humans and marine predators. To feed, the starfish climbs atop living coral colonies and everts (extrudes) its entire stomach through its mouth directly onto the coral, secreting caustic digestive enzymes that liquefy the delicate coral polyps within their limestone cups before absorbing the pre-digested slurry. It has extraordinary regenerative power, regrowing severed arms easily.
Behavior and Ecology
Crown-of-thorns starfish are predominantly nocturnal feeders in healthy reefs, sheltering beneath coral overhangs during the day and emerging at nightfall to feed; however, during massive population outbreaks, starved starfish feed brazenly in broad daylight. An adult starfish can devour up to 60 to 100 square feet (6 to 10 square meters) of living hard coral per year. Reproduction is intensely prolific: during summer spawning, a single large female can release an astonishing 60 to 100 million eggs into the water column, creating vast larval clouds that drift across ocean currents.
Conservation and Human Impact
Not evaluated on the IUCN Red List due to its hyper-abundance and outbreak dynamics, Acanthaster planci is the focus of intensive marine management and control programs across Australia and Southeast Asia. On the Great Barrier Reef, teams of specialized divers manually inject invasive starfish with lethal doses of bile salts or vinegar to suppress outbreaks, while conservationists work to reduce terrestrial agricultural nitrogen runoff that fuels boom-and-bust larval blooms.
Interesting Factoid
The crown-of-thorns starfish eats hard coral by vomiting its entire stomach out of its mouth directly onto the coral, turning the living coral animals into soup before slurping them back inside.