The sound of the world’s largest parrot fish swimming toward him， says Douglas McCauley， is not some watery swish， swish. It’s crunch， crunch. “You can hear a school of them before you see it，” he says.

Bumphead parrot fish （Bolbometopon muricatum） grow to “about the size of a junior high school kid” as McCauley puts it. And feeding is a noisy business because they eat -- and loudly digest -- what’s essentially rock.

The fish gouge out hunks of reef and snap thumb-sized coral branches. But what McCauley finds even more impressive are the noises of the parrot fish’s down-deep throat teeth， which can grow wider than half dollars， milling the coral chunks.

Crushing coral uncovers what the fish really want： fleshy polyps and other tiny organisms hiding inside. Bumpheads excrete the broken-up coral as gravel and a plume of white sand that “just hovers，” McCauley says， “as if you had opened a carton of milk underwater.”

So prodigious a grinder of coral is the bumphead that more than four tons of coral sediment land on the reef in a year from the excretions of a single parrot fish.

Calculating that number required 130 days underwater from McCauley， now at the University of California， Santa Barbara， and a series of helpers carrying syringes the size of turkey basters for collecting excreted coral.