Bumphead parrot fish declare their arrival with a crunch

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. On two atolls in the Northern Line Islands in the Pacific, McCauley eased as close as he could to one bumphead at a time and recorded what it ate and pooped as long as he could keep it in sight. On his best day, that was 5.3 hours. If he took his eyes off a fish for just 30 seconds, he could lose it. “I kept a PowerBar in my sleeve,” he says.

His hard-earned data reveal a dilemma for people who love coral reefs. Bumpheads can help corals (for example, by mowing down smothering algae) but in crowds over decades they may reduce the diversity of coral species, McCauley and his colleagues suggest July 26 in Conservation Biology. And the charismatic fish themselves rank as vulnerable to extinction, a conservation conundrum with a terrestrial parallel. The big, loud, wonderful, alarming bumpheads, McCauley says, are “underwater elephants.”