Corals are living organisms, not plants or rocks, forming complex colonies of tiny individual animals called polyps. These polyps, often just a few millimeters in diameter, secrete calcium carbonate to build the hard skeletons that create coral reefs. Coral reefs provide habitat for a quarter of all marine species, support fisheries, and protect coastlines from wave action. Despite their seemingly robust structure, corals serve as a food source for a variety of marine animals.
Fish That Consume Coral
A diverse array of fish species directly consumes coral. Parrotfish are examples, known for their continuous grazing on reefs. These fish consume not only the algae growing on corals but also the coral polyps and the calcium carbonate skeleton itself. Their feeding action helps prevent algal overgrowth, which can smother corals, and contributes to the creation of fine sand that forms beaches and seafloors.
Butterflyfish also include many species that feed on coral, with some being obligate corallivores, meaning live coral polyps can constitute up to 80 percent of their diet. Other butterflyfish are facultative corallivores, diversifying their diet with other foods while still consuming coral. Their feeding habits range from generalists, consuming a wide array of food, to specialists that focus on a limited selection of corals. These fish often employ their specialized mouths to pick individual polyps from the coral surface.
Invertebrates That Prey on Coral
Beyond fish, several invertebrate species feed on corals. The crown-of-thorns starfish (COTS) is a notable coral predator found throughout the Indo-Pacific. This starfish consumes hard coral polyps. A single adult COTS can consume up to 10 square meters of coral per year, and outbreaks of these starfish can cause significant coral loss, sometimes stripping up to 90 percent of coral tissue from affected reefs.
Certain snail species, such as those in the genus Drupella, also prey on coral tissue. These snails graze on corals, stripping the tissue from the skeleton and leaving behind distinctive white feeding scars. Drupella snails often prefer fast-growing branching corals like Acropora and Pocillopora, but will consume other species if their preferred prey is scarce. Outbreaks of Drupella can lead to substantial coral mortality and reduced reef resilience.
Sea urchins primarily consume algae and organic material, contributing to reef cleanliness by preventing algal overgrowth. However, certain sea urchin species can graze on coral, especially when other food sources are limited or their populations become unusually dense. In such instances, their grazing activity can lead to the erosion of coral structures.
Unique Adaptations for Coral Consumption
Coral-eating animals possess specialized adaptations for their unique diet. Parrotfish, for instance, have fused teeth that form a beak-like structure, which allows them to scrape algae and bite off pieces of coral. Many species also have a second set of grinding teeth, called pharyngeal teeth, located in their throats, which further break down the ingested calcium carbonate skeleton. This continuous grinding action means their teeth are constantly wearing down and regrowing.
Butterflyfish exhibit elongated snouts and small, forceps-like mouths that are suited for probing into coral crevices and carefully picking out individual coral polyps. This precision feeding allows them to access the soft tissues without damaging the hard coral skeleton extensively.
The crown-of-thorns starfish utilizes an eversible stomach. It pushes its stomach out through its mouth and directly onto the coral, releasing digestive enzymes externally to liquefy the coral’s tissues before absorbing the resulting “coral soup”. Coral-eating snails, such as Drupella species, use a specialized, ribbon-like tongue called a radula to rasp tissue from the coral skeleton. This abrasive tool enables them to remove the coral polyps, leaving behind a scarred surface.