Corals are living animals, despite their often rock-like appearance, and they form the structural foundation of vast marine ecosystems known as coral reefs. These complex underwater cities provide shelter and food for countless marine species. While corals are builders, they also serve as a food source for various marine creatures. This article will explore the diverse animals that consume coral, detailing their methods of consumption, and examining the broader ecological implications for coral reefs.
Animals That Eat Coral
Many marine animals consume coral, including fish and invertebrates. Parrotfish, identifiable by their vibrant colors and beak-like jaws, primarily feed on algae that grow within coral polyps and on the coral surface. To access this algae, they scrape off bits of the hard calcium carbonate skeleton. Butterflyfish, another group of reef fish, often have elongated snouts adapted for picking individual coral polyps or the soft tissues covering the coral skeleton. Some butterflyfish species are obligate corallivores, meaning their diet consists almost entirely of live coral polyps.
Invertebrates also consume coral. The Crown-of-Thorns starfish (Acanthaster planci) is a well-known coral predator, particularly in the Indo-Pacific region. These starfish evert their stomachs over the coral to digest the polyps externally. Certain marine snails, such as those in the genera Drupella and Coalliophila, also feed on live coral by stripping tissue from the skeleton, leaving noticeable white scars. Some species of nudibranchs, or sea slugs, are known to prey on live coral, often specializing in particular coral types.
How Corallivores Consume Coral
The mechanisms by which corallivores consume coral are diverse, reflecting their varied feeding strategies and adaptations. Parrotfish employ a scraping and biting action using their fused, beak-like teeth. This allows them to scrape off coral tissue and the underlying skeleton. After ingestion, they digest the organic matter and excrete the finely ground coral skeleton as sand, contributing to reef sediment.
Starfish, such as the Crown-of-Thorns, utilize an external digestion method. They extend their stomach out through their mouth and over the coral surface, releasing digestive enzymes directly onto the coral polyps. This process breaks down the coral tissue, allowing the starfish to absorb the liquefied nutrients. Butterflyfish, with their narrow snouts and small mouths, nip and pick at individual coral polyps or the thin layer of tissue covering the coral skeleton. This method allows them to selectively consume the soft parts of the coral without extensively damaging the underlying structure. Some marine snails, like the Drupella species, use a radula to rasp away coral tissue or even bore into the coral structure to reach the polyps.
Ecological Role of Coral Eating
Coral eating is a natural process with ecological importance within healthy coral reef ecosystems. Some corallivores, like parrotfish, contribute to bioerosion, the breakdown of coral skeletons. Their feeding creates fine sand and sediment, which are components of reef environments and beaches. This bioerosion also helps prevent certain fast-growing coral species from dominating the reef, promoting overall coral diversity.
The consumption and excretion of coral tissue by corallivores also play a role in nutrient cycling on the reef. As these animals process coral, they release nutrients back into the water column, which can then be utilized by other reef organisms. This contributes to the overall productivity and health of the ecosystem. Many coral-eating animals are important for reef balance and resilience, showing that not all coral consumption is detrimental.
When Coral Predation Becomes Harmful
While coral consumption is a natural reef process, certain circumstances can cause it to become destructive. Outbreaks of the Crown-of-Thorns starfish (COTS) represent a threat to coral reefs, particularly in the Indo-Pacific. During these outbreaks, COTS populations explode, leading to consumption of coral over large areas. These surges can be triggered by natural factors, but human activities like nutrient runoff from land, which can increase COTS larvae survival, and overfishing of COTS predators (such as triggerfish and giant triton snails) can exacerbate their frequency and intensity. The resulting devastation can take years or even decades for reefs to recover.
Corals already facing stress from other environmental factors are more susceptible to predation. Climate change impacts, such as coral bleaching due to rising ocean temperatures and ocean acidification, weaken corals, making them more vulnerable to even normal levels of consumption. Pollution and disease can further compromise coral health, reducing their ability to recover from feeding damage. Human influences, including the removal of predators that naturally control corallivore populations, can disrupt the balance of the reef ecosystem. This imbalance can lead to increased pressure on corals, transforming a typically beneficial interaction into a detrimental one for the reef.