Parrotfish: Coral Friend or Foe?

A parrotfish is a lovely sight to behold on a dive, and indeed, it does quite closely resemble its avian namesake. In addition to its bright colors and varied marking patterns, the teeth of the parrotfish are fused together in a way that looks remarkably like a parrot beak. Its jaws are so strong that it can bite through rock and calcified coral structures! Evidence of parrotfish feeding is seen on corals by way of missing chunks, causing speculation that parrotfish are actually to blame for the declining health of coral reefs. Despite their indiscriminate feeding habits, parrotfish actually play an important role in the survival of coral reefs and tropical islands.

Parrotfish are generally found in shallow tropical waters, no deeper than 70 meters. They inhabit coral reefs, as well as seagrass beds and other rocky coastal areas. The primary food source for parrotfish is algae, which is what causes them to graze on corals, sometimes resulting in large chunks missing from a coral colony. Coral polyps maintain a symbiotic relationship with algae in order for both organisms to feed. CO2 produced by the coral is consumed by the algae for the process of photosynthesis, which creates sugars and amino acids as nutrients for the coral. This cycle creates the beautiful coloration that corals exhibit in shallow reefs. But ironically, an overgrowth of algae will actually suffocate the polyps, which leads to the bleaching death we’re seeing so often lately.

Excessive algae growth on coral

Unchecked algae growth is not only detrimental for corals and seagrasses, it can spell doom for many other ocean species as well. Decomposition of algae consumes much of the dissolved oxygen in the water column that marine plants and animals need to survive. Although some coral and grasses are consumed in their quest for algae, without the constant grazing of the parrotfish, algae populations would explode, choking entire ecosystems as they rapidly reproduce.

Another beneficial side effect of the parrotfish’s consumption of coral is the way their system processes and expels coral. Rocks and coral are ground up by teeth found in the throat of the parrotfish, and the waste is excreted as sand that partially comprises the soft beaches of many tropical islands, which, as they build up, begin propagation of new plants and other life forms. The excreted sands on the shallow ocean floor make up the foundation for new coral formations to build. It would seem as though the damage is mitigated by the regeneration that comes as a result of the insatiable appetite for algae the parrotfish have.

Parrotfish grazing on coral algae

Moreover, parrotfish rely on coral reefs for the survival of their own species. They are pelagic spawners, meaning they release their eggs into the water column, where they drift until settling into the coral to hatch. Coral reefs are important nurseries for parrotfish, so it is not in the interest of the parrotfish to indiscriminately destroy the very grounds in which they are born. As with all ecosystems, each member serves a vital function to maintain the health of the entire reef, and despite the bad press, it looks like the parrotfish is simply doing its job.

Photos via laszlo-photo, derekkeats, eutrophication&hypoxia