Fish that Eat the Coral Reef
Coral reefs are by far the largest animal-built structures on Earth. The Great Barrier Reef stretches for nearly 1,500 miles (2,400 km) and is even visible from space. Nevertheless, most of the coral reef is dead, composed of the skeletons of individual polyps. Only the superficial layer of the reef is alive and this living skin represents a nutritious source of food for any animal that is able to exploit it. During the day, when many reef animals are grazing, the polyps withdraw to the protection of their limestone shelter, but this is not always enough to guarantee their safety.
How do parrotfish get past the coral's defenses?
Parrotfish are so-called because their fused teeth give their mouths a beaklike appearance. These teeth are situated outside the jaw bones, so the beak protrudes beyond the mouth. This is perfect for scraping algae from the surface of rocky substrates, but can also get past one of the algae's defenses -- growing within the matrix of the coral itself. In some species, such as the hump-headed parrotfish, the beak can take a chunk out of the reef itself. Interestingly, although the parrotfish eat the polyps themselves, these herbivorous fishes are probably primarily Interested in the zooxanthellae contained within the coral's tissues, rather than the coral itself.
How do the parrotfish eat such a rocky diet?
To counteract their tough diet, parrotfishes' teeth grow continuously. But those that form the beak are not the only teeth that these remarkable fish have; the platelike pharyngeal teeth towards the back of the mouth can bring considerable crushing force to bear, pulverizing even the tough limestone. After this, the coral's resistance is at an end. In the fish's gut, living tissue is separated from the limestone rubble and powder.This ground material is ejected by the parrotfish as fine, white grains, which makes up a considerable proportion of the highly prized white sand found in coral reef lagoons and beaches!
What other predators threaten the coral?
Although the damage done by parrotfish can be quite dramatic, the coral faces a multitude of other biological threats. Many different invertebrates target the reef, including coral-eating whelks, fireworms, and nudibranchs. In the latter case, the corallivores can be hard to spot -- they are and sometimes accidentally introduced into a reef aquarium. Many starfish also feed on the coral, stripping away the living polyps as they move across the surface of the colony and leaving a white stripe behind them.
Perhaps the most notorious is the crown-of-thorns starfish (Acanthaster plonci), which may-grow to almost 20 inches (half a meter) across. This creature has wreaked havoc in many areas of the world most notably, perhaps, on the Great Barrier Reef. Like all starfish, it feeds by everting its stomach and dissolving its food -- in the case the living polyps -- with digestive enzymes. An outbreak of crown-of-thorns starfish presents a serious problem for reef communities, as in high densities they can kill even mature, well-established coral colonies. They do have enemies, including the giant triton (a snail), but until the exact cause of the recent population explosions of this starfish is established, the outlook is bleak for many reefs.