ParrotfishesScaridae

A reef is being taken apart at close to the rate it is being built, and parrotfish do most of the taking apart. Coral lays limestone down. Parrotfish bite it off again, all day, every day, and the difference between those two rates is whether a reef gains height, holds level, or slowly loses it. Nothing else on the reef removes rock on that scale.

The family does the job in three different ways, and the difference is the depth of the bite. Excavators, the heavy blunt-headed species, close the jaws at close to a right angle and take a real bite out of the substrate, leaving craters in dead limestone and sometimes in living heads. Scrapers, which covers most of the species a diver meets here, work at a shallower angle and lift the surface off without breaking far into it. Browsers barely touch stone at all, cropping fleshy weed and seagrass leaves and leaving the mineral part behind. Three fish that look like the same animal in different colors are running three separate quarries.

The rock does not leave the reef. Ground fine and passed as sediment, some of it ends up on the beach and some of it settles into the cavities inside the reef framework, where infill and cement are part of what turns a loose stack of coral skeletons into solid rock. What the bite leaves behind matters as much: bare, clean, grazed limestone is the kind of ground a coral larva can settle on.

Eighteen species are listed here. Reef scientists keep the whole thing as a budget, carbonate added against carbonate removed, and the excavators sit on both sides of it at once, which is why the answer is never obvious and why the biggest parrotfish, the slow ones that are easiest to fish out, matter more than their numbers suggest.