Starfish, or sea stars, are invertebrates belonging to the phylum Echinodermata, known for their radial symmetry and ability to regenerate lost limbs. Coral reefs are massive structures built by tiny animals called coral polyps, which secrete hard calcium carbonate skeletons. While most starfish species are scavengers, a few are dedicated predators that target these reef-building organisms. The feeding habits of certain species pose one of the most serious biological threats to tropical coral ecosystems worldwide.
Identifying the Primary Predator
The vast majority of the 1,900 known starfish species do not consume living coral, but one group, the Crown-of-Thorns Starfish (CoTS), is an obligate corallivore. This predator, represented by species like Acanthaster planci and Acanthaster solaris, is native to the Indo-Pacific region and is recognized as a major driver of coral loss. The CoTS is one of the largest starfish, capable of growing up to one meter in diameter, and possesses 10 to 21 arms radiating from its central disc.
Its name comes from the hundreds of sharp, venomous spines that cover its upper surface, providing a potent defense against most predators. CoTS are highly mobile and have an enormous appetite for hard coral polyps. Unlike other starfish, whose diets are varied, the adult CoTS relies almost exclusively on the soft tissue of reef-building corals for its survival.
The Unique Mechanism of Coral Consumption
The Crown-of-Thorns Starfish employs a feeding mechanism known as stomach eversion. To begin feeding, the starfish climbs onto the surface of a coral colony and pushes its entire cardiac stomach out through its mouth, which is located on its underside. This everted stomach tissue then completely envelops the living coral surface.
Once the stomach is extended, the starfish releases powerful digestive enzymes directly onto the coral polyps. These enzymes break down the soft tissue externally, liquefying the coral’s organic matter. The starfish then retracts its stomach, absorbing the resulting nutrient-rich liquid. This external digestion process leaves behind a white skeleton, which is the only visible sign of the consumed coral. An adult CoTS can consume an area of coral roughly equal to its own body size every day, leaving a trail of destruction as it moves across the reef.
Ecological Consequences of Mass Outbreaks
While the Crown-of-Thorns Starfish is a natural component of coral reefs, population explosions, known as outbreaks, severely impact the ecosystem. An outbreak is officially declared when densities exceed approximately 15 individuals per hectare, at which point coral consumption far outpaces natural growth rates. Outbreaks have been responsible for a significant portion of coral mortality on major reef systems, such as the Great Barrier Reef, where they account for about 40% of the coral cover decline between 1985 and 2012.
During these mass events, a single starfish can consume up to 10 square meters of live coral tissue in a year. The rapid, widespread predation severely reduces coral cover, particularly targeting fast-growing species like Acropora, which are important for reef structure and fish habitat. The resulting loss of habitat and structural complexity leads to a significant reduction in overall reef biodiversity.
The destruction caused by a CoTS outbreak is often compounded by other environmental stressors. When an outbreak occurs shortly after a mass coral bleaching event, the starfish preferentially consumes the few remaining corals that survived the heat stress. This predation removes the most resilient coral colonies, hindering the reef’s ability to recover and adapt to future warming. This interaction can accelerate a phase shift, where the ecosystem moves from being coral-dominated to an area overgrown by algae and soft corals.
Managing Reef Health and CoTS Populations
Mitigating the impact of Crown-of-Thorns Starfish outbreaks requires management efforts. The most effective strategy involves manual culling, where trained divers systematically locate and neutralize the starfish. This approach is highly focused and is typically targeted at protecting high-value or strategically important reefs.
The primary culling method uses a single-shot injection of a lethal solution into the starfish’s body. Effective and eco-friendly solutions include ox bile salt or common household vinegar. A single injection of 25 milliliters of vinegar, for instance, results in 100% mortality within 48 hours without harming the surrounding coral or marine life. Monitoring programs, such as the “manta tow” survey method, are used to detect early signs of an outbreak, allowing management teams to deploy culling vessels before the population reaches destructive levels.