Corallivory, the consumption of coral, is a fundamental and ancient interaction within reef systems. This behavior involves diverse marine life, including hundreds of species of fish, echinoderms, mollusks, and worms, that consume the living tissues or skeletal structures of coral colonies. While the term might suggest destruction, this predation is a natural and necessary component of a functioning coral reef ecosystem. Corallivores regulate coral population dynamics and maintain the physical structure of the reef environment.
Feeding Mechanisms of Coral Eaters
The way a corallivore consumes coral dictates its impact, leading to classification into distinct feeding groups. One major group consists of polyp feeders or browsers. They use specialized jaws or mouthparts to graze only the soft, nutrient-rich tissue of the coral. These animals leave a visible feeding scar on the hard skeleton but do not actively break the underlying calcium carbonate structure. The coral tissue can often regrow and cover the scraped area.
A second, more physically destructive category is the skeletal excavators and scrapers. These organisms are equipped with powerful teeth or plates capable of biting into the hard coral skeleton. Scrapers remove the coral tissue along with a thin layer of skeleton, while excavators take large, deep chunks of the calcium carbonate material. This process is effective for nutrient extraction, as the animals process the ground-up skeleton to access trapped organic material.
The third method is practiced by sucking or extruding feeders, which are primarily invertebrates that do not rely on scraping or biting. A prime example is the Crown-of-Thorns Starfish (Acanthaster spp.). It everts its stomach over the coral colony and releases digestive enzymes directly onto the surface. The starfish then liquefies the coral tissue and sucks up the resulting organic slurry, leaving a stark white, denuded skeleton. Some corallivorous snails similarly use enzymes to break down tissue before consuming it.
Major Groups of Corallivores
Corallivory is dominated by two primary taxonomic groups: fish and invertebrates. Among the fish, parrotfish are significant players, recognized as excavators and scrapers that consume vast amounts of stony coral. Species like the Green Humphead Parrotfish possess fused teeth that form a powerful beak, allowing them to grind up coral skeletons. This grinding process produces a substantial portion of the fine, white sediment that forms sandy beaches in tropical areas.
Butterflyfish are another visible group, with approximately 50% of their species feeding on coral to some degree. These fish are browsers, using their narrow snouts and small, brush-like teeth to nip off individual coral polyps. Many are obligate corallivores, relying on coral for more than 80% of their diet, making their populations dependent on healthy coral cover.
In the invertebrate realm, the Crown-of-Thorns Starfish (COTS) is the most notorious corallivore, capable of consuming an area of live coral equal to its body diameter daily. Certain snails and worms also contribute, such as the Drupella snails, which feed on the tissues of branching corals like Acropora. These invertebrates often target specific, fast-growing coral genera such as Acropora, Pocillopora, and Porites, influencing which coral species thrive on a reef.
Corallivory’s Impact on Reef Structure
The consumption of coral tissue and skeleton has a dual ecological significance, acting as both a constructive and destructive force on the reef’s physical structure. Under normal conditions, natural corallivory provides a beneficial role by helping to maintain the reef’s overall diversity and health. Excavating parrotfish contribute to bioerosion by removing dead and live coral skeleton, which constantly reshapes the reef topography and prevents the structure from becoming too dense.
The removal of coral tissue and skeleton creates open patches on the reef surface, available for the settlement of new coral larvae. This disturbance prevents dominant coral species from monopolizing the space, ensuring a mosaic of different species and higher biodiversity. The constant turnover of material through corallivory also contributes to nutrient cycling within the reef system.
However, when corallivore populations experience explosive growth, the balance shifts to a destructive role, leading to severe ecological damage. The mass outbreak of the Crown-of-Thorns Starfish is the most dramatic example, where large aggregations consume a significant percentage of live coral cover rapidly. This tissue loss forces the reef into a degraded state, reducing structural complexity and habitat for other species. The resulting bare skeleton is often colonized by fast-growing algae, leading to a harmful phase shift dominated by algae rather than coral.
How External Factors Disrupt Reef Balance
Although corallivory is natural, human activities disrupt the delicate balance between coral and its predators, exacerbating negative impacts. Overfishing is a major driver of this imbalance, as it removes the natural predators of invertebrate corallivores, such as triggerfish, pufferfish, and sea turtles that consume COTS. The resulting release from predation pressure allows populations of invertebrates, like COTS and Drupella snails, to increase unchecked, leading to outbreak conditions.
Nutrient runoff and pollution from land-based activities, such as agriculture and sewage discharge, also contribute by fueling the planktonic larvae of corallivores. The increased availability of phytoplankton, the primary food source for COTS larvae, boosts larval survival rates and leads to massive recruitment events. These events trigger devastating outbreaks that destroy vast areas of live coral.
Furthermore, climate change acts as an additional stressor, making corals more susceptible to predation. Elevated ocean temperatures cause corals to expel their symbiotic algae (bleaching), which depletes their energy reserves. A stressed, weakened coral is less able to heal a predation wound, meaning that even normal levels of corallivory can become lethal. The combination of weakened corals and increased corallivore populations accelerates reef decline.