CORALS ARE THE reef builders, and parrotfish the demolishers, and that makes them both super-important. By crunching coral into sand that washes up on beaches, parrotfish help islands rise.
What they do is called bioerosion. On the Great Barrier Reef up to 9 kilograms per square metre of reef is removed each year by their powerful teeth, operating within those parrot-like beaks. Some parrotfish species take as many as 20 bites per minute. A bumphead parrotfish (Bolbometopon muricatum) can excrete 90 kilograms of sand in a year. The coral they consume is sometimes dead, and includes old debris, so they aren’t as destructive as consumption rates suggest.
(Image Credit: Wikimedia)
Some parrotfish species browse seagrasses and seaweeds rather than anything hard. Those that matter here are the ‘scrapers’ and especially the ‘excavators’. Scrapers rasp away at corals and coralline algae, while excavators take bites. What they are digesting is not proven, but recent research implies that protein-rich bacteria and other microbes living among coral are their targets, rather than algae or coral, as was once thought.
In the Maldives, where the tides are too gentle to grind up coral debris, scientists believe that most of the seashore sand has come out of parrotfish. Think about that. You are relaxing on a tropical beach, and what you are reclining on is fish faeces, more or less.
(Image Credit: Laszlo Ilyes)
On many reefs around the world, parrotfish are heavily fished, inviting concerns about future sand supplies. But parrotfish aren’t the only bioeroders. Some pufferfish bite off the tips of staghorn corals, although the damage they do is trivial.
Other agents of bioerosion include sea urchins, sponges, hermit crabs, shrimps, barnacles, bivalves, worms, bacteria and algae. Tropical reefs are so rich in life there are inevitably many coral consumers, though none matches parrotfish for impacts. Some of them benefit when bleaching yields weakened and dead coral.
What they do is nothing new. Parrotfish proliferated just a few million years ago, but bioerosion has been detected in fossil reefs living more than 500 million years ago.