Fish
Parrotfish
Scaridae · also called parrotfish, parrot fish
Parrotfish are the reef’s gardeners and its sand mills. All day they scrape algae off coral rock with a beak strong enough to bite limestone, and all day they pass the ground-up rock out the other end as clean white sand. On many tropical islands, the beach you are standing on came out of a parrotfish.
Overview
Parrotfish — family Scaridae, about 100 species — live on coral and rocky reefs and seagrass beds throughout the world’s warm seas. They are usually brightly coloured (the name comes from the beak and the colours), active by day, and often seen in loose groups moving across the reef, their bites making an audible crunching sound underwater. Most feed on the film of algae and microorganisms growing in dead coral rock, biting off pieces of the substrate to get it.
Beak and mill
A parrotfish’s teeth are fused into a beak made of one of the hardest, most fracture-resistant biominerals known — hard enough to gouge solid limestone. Swallowed rock is then crushed by a second set of teeth in the throat, the pharyngeal mill, and the indigestible carbonate is excreted as fine sand. A large parrotfish can produce hundreds of kilograms of sand a year, and much of the white sand on tropical reefs and beaches is exactly this.
Reef engineers
By grazing algae relentlessly, parrotfish keep seaweed from smothering corals and help reefs stay coral-dominated and bounce back after damage; their bites also clear space for new coral to settle. Reefs stripped of parrotfish by fishing tend to become algae-covered with little live coral.
Strange biology
Most parrotfish start life female and later become male, switching from a drab “initial phase” to a vivid “terminal phase.” And many species sleep each night inside a cocoon of mucus they secrete, which appears to hide their scent from night-hunting moray eels and to deter parasites.
Status
Most parrotfish are Least Concern and common. The giant green humphead parrotfish is Vulnerable, easily speared at night on its sleeping ledges and slow to mature. Because of their role as grazers, several countries and reserves now protect parrotfish specifically as a reef-management measure.
Parrotfish: common questions
Do parrotfish really make sand?
Yes, and in large amounts. A parrotfish feeds by biting chunks off the reef to get at the algae and microbes growing in and on the coral rock. It cannot digest the rock, so it grinds it up with a second set of teeth in its throat and passes the pulverised limestone out as fine, pale carbonate sand. Estimates vary by species and size, but a large parrotfish can produce on the order of hundreds of kilograms of sand per year, and on many tropical reefs and beaches the majority of the white sand — sometimes the great majority — is parrotfish excrement. The famous white beaches of the Maldives and parts of Hawaii are substantially parrotfish-made.
How can a fish bite solid coral?
The parrotfish's teeth are fused into a solid beak, and the tooth material is exceptionally hard and stiff — among the most fracture-resistant biominerals known, with the crystals woven together and getting harder toward the biting edge. Behind the beak, powerful jaw muscles and the throat mill let the fish scrape, gouge, and crush limestone all day, every day. The beak wears down and grows continuously to compensate. Engineers have studied parrotfish tooth structure for clues to making tougher ceramics.
Why are parrotfish important for coral reefs?
They are among the most important grazers on a reef. By constantly cropping algae, parrotfish stop fast-growing seaweed from overgrowing and smothering corals, which keeps the reef in a coral-dominated state and helps it recover after bleaching or storms. Their bite marks also clear bare patches where new coral larvae can settle, and their sand production builds and replenishes beaches and reef flats. Reefs that lose their parrotfish — usually to overfishing or spearfishing — often shift to being carpeted in algae, with much less live coral.
Why do parrotfish sleep in a bubble of mucus?
Many parrotfish species secrete a transparent cocoon of mucus from glands near the gills and settle inside it in a crevice each night. The leading explanation is that the mucus envelope blocks the fish's scent from drifting out, hiding it from predators such as moray eels and nocturnal sharks that hunt by smell in the dark, and it may also act as a barrier against blood-sucking parasites like gnathiid isopods. Building the cocoon takes the fish up to an hour and uses a noticeable share of its daily energy, which suggests the protection is worth a lot.