The Crown-of-Thorns Starfish (COTS) is a marine invertebrate distinguished by its spiky appearance. It inhabits Indo-Pacific coral reefs, from the Red Sea to the west coast of the Americas. Its feeding habits have significant implications for reef health.
Primary Food Source
Adult Crown-of-Thorns Starfish primarily eat hard, or stony, coral polyps. These corals form the structural foundation of coral reefs, and the starfish is a corallivorous predator. While COTS are generalist feeders within the hard coral category, they prefer certain types, such as fast-growing branching and table-like corals like Acropora and Pocillopora species.
They often avoid more rounded corals, like Porites species, which may have less exposed surface area or host protective symbionts. During periods of low hard coral density or outbreaks, COTS may also consume soft corals, though hard corals remain their main food source. Juvenile starfish, however, begin as herbivores, feeding on encrusting algae found on dead coral rubble and other concealed surfaces.
How the Starfish Consumes Its Prey
The Crown-of-Thorns Starfish consumes coral polyps using a distinct feeding mechanism. It uses its numerous tube feet to climb onto a living coral colony, fitting closely to the coral’s surface. The starfish then extrudes its cardiac stomach out through its mouth, spreading it over the coral tissue. This everted stomach can cover an area nearly as wide as the starfish itself.
Digestive enzymes are secreted from the stomach surface directly onto the coral polyps, breaking down the coral tissue externally. The starfish then absorbs the liquefied nutrients. This process leaves behind a white, denuded coral skeleton. An individual starfish can consume a significant amount of coral, with estimates ranging from 6 to 10 square meters of living coral per year.
Ecological Ramifications of Its Diet
The feeding habits of the Crown-of-Thorns Starfish impact coral reef ecosystems. Their consumption of coral polyps directly leads to the loss of coral cover and the degradation of reef structures. The complex three-dimensional habitats provided by corals are destroyed, affecting marine life that depend on them for shelter and food. This can result in declines in fish populations, particularly those species that are obligate coral feeders.
When COTS populations experience rapid increases, known as “outbreaks,” the destructive effects are amplified. Outbreaks can transform reefs into barren landscapes, with coral cover dropping in a relatively short period. For instance, some studies have observed coral cover declining by over 40% in just four years during an outbreak. Such widespread coral mortality reduces the reef’s resilience, making recovery more challenging and prolonged.
Factors Influencing Feeding Behavior
Several factors influence the feeding patterns and diet choices of the Crown-of-Thorns Starfish. The availability and density of preferred coral species, such as Acropora, significantly impact their feeding rates and movement across the reef. When preferred corals are abundant, COTS may have higher feeding rates. Conversely, if preferred coral is scarce, they may exhibit dietary flexibility, consuming less preferred species or even algae.
Environmental conditions also play a role in their feeding behavior. Water temperature affects their metabolic rate, with higher temperatures potentially increasing their feeding and locomotion rates.
Human activities can indirectly influence COTS populations and their feeding patterns. Increased nutrient runoff into coastal waters can lead to phytoplankton blooms, which provide more food for COTS larvae, potentially enhancing larval survival and contributing to outbreaks. Additionally, overfishing of COTS predators may reduce natural controls on starfish populations, further exacerbating outbreak frequency and intensity.