The Crown-of-Thorns Starfish (COTS), known scientifically as Acanthaster planci, is a large, multi-armed sea star native to the Indo-Pacific region. As one of the ocean’s most voracious coral predators, the COTS is a major contributor to coral loss across tropical reefs, including the iconic Great Barrier Reef. This invertebrate is recognized worldwide for its destructive feeding habits on coral polyps.
Anatomy and Unique Biological Features
The COTS is physically distinctive and can grow quite large, reaching up to 80 centimeters in diameter. Unlike the typical five-armed sea star, an adult COTS can possess anywhere from 12 to 21 arms radiating from its central disc. These arms and the entire upper body surface are covered in numerous sharp, needle-like spines that can reach lengths of four to five centimeters.
These spines contain a saponin-based venom, serving as an effective defense mechanism against most potential predators. The starfish’s mobility is powered by hundreds of tube feet on its underside, allowing it to move across the reef at a relatively quick pace.
The COTS is highly destructive to corals due to its unique feeding process. When feeding, the starfish extends, or everts, its stomach out through its mouth and over the surface of a coral colony. The stomach then secretes powerful digestive enzymes onto the living coral polyps, liquefying the tissue externally. The starfish absorbs this nutrient-rich “coral soup” before retracting its stomach, leaving behind a white, denuded coral skeleton.
The Engine of Outbreaks: Reproduction and Population Dynamics
The destructive potential of the COTS is tied directly to its incredible reproductive capacity, which drives dramatic population surges known as outbreaks. Female starfish are highly fecund, with a single large individual capable of producing tens of millions of eggs per year, sometimes exceeding 200 million. Reproduction occurs through synchronized mass spawning, where males and females release gametes into the water column for external fertilization, typically during the warmer summer months.
The resulting larvae are planktonic, drifting in the water for up to a month and feeding on microscopic algae called phytoplankton. Larval survival rates are hypothesized to be significantly enhanced by increased nutrient runoff from adjacent land, often agricultural or sewage waste. This runoff effectively fertilizes the ocean water, causing massive phytoplankton blooms that provide an abnormally rich food source for the developing COTS larvae.
A small increase in larval survival rate can translate into a massive recruitment event of juvenile starfish onto the reef. While outbreaks are a natural, cyclical phenomenon, human-caused nutrient enrichment is thought to increase their frequency and severity beyond historical norms. The removal of natural predators through overfishing is another factor believed to reduce the natural control mechanisms on COTS populations.
Ecological Impact: A Major Coral Predator
The immense appetite of the COTS, especially during an outbreak, makes it a leading cause of coral loss in the Indo-Pacific. An adult starfish can consume up to 10 square meters of living coral tissue annually, leaving behind distinctive white feeding scars. The starfish preferentially targets fast-growing, branching, and plating corals, particularly species from the genus Acropora, which are the primary reef-building corals.
When COTS populations reach outbreak densities, exceeding 10 to 15 adults per hectare, they can overwhelm a reef and consume coral faster than it can regenerate. Severe outbreaks can result in the loss of 90% or more of a reef’s live coral cover. These events dramatically shift the ecological balance, often resulting in a phase shift where the coral-dominated reef structure is replaced by a less diverse, algae-dominated substrate.
The Great Barrier Reef, for example, has experienced four major outbreaks since the 1960s, with each one compounding the damage from other disturbances like coral bleaching and tropical cyclones. The recurring nature of these outbreaks reduces the time available for coral recovery, which can take a decade or more under ideal conditions.
Controlling Crown-of-Thorns Starfish Populations
Management of COTS populations largely focuses on active intervention and localized culling programs, particularly in areas of high tourism or ecological value. The most common active control method involves divers physically locating and injecting the starfish with a lethal substance. Historically, this required multiple injections of a toxic solution, but modern techniques now use a single-shot injection of agents like vinegar or bile salts (ox bile).
The bile salts work by dissolving the starfish’s cell membranes, causing rapid death within 48 hours, and have been certified as safe for the surrounding reef environment. While manual culling can be highly effective at protecting specific reefs and reducing localized populations, it is resource-intensive and cannot control a widespread outbreak across an entire reef system. Robotic systems have also been developed to autonomously detect and inject the starfish, increasing the efficiency of culling efforts.
Biological control, such as the natural predation by the giant triton snail (Charonia tritonis), is generally ineffective for managing large-scale outbreaks, as the triton snail is rare and slow-feeding. Preventative measures, though more complex, address the root cause by focusing on improving water quality. Reducing agricultural runoff and nutrient pollution is a long-term strategy aimed at limiting the abundance of phytoplankton, thereby lowering the survival rate of COTS larvae and reducing the frequency and intensity of future outbreaks.