A single fish can chew its way through a staggering five tonnes of coral a year.
Smaller parrotfish, colourful cousins of the bumphead,are also at it.
But they all give a little back
as the ground-up coral comes out as sand.
This is island building of a different kind.
So some of the Pacific's most elegant beaches
have sprung from a less than stylish birth.
Above the water, time seems to stand still.
But the Pacific islands are always changing, if slowly.
This is Bora Bora,
a volcano in what could be called "late middle age".
The sloping flanks have slipped further into the sea,
pushing the reef away from the shore.
A lagoon is formed.
It's a patchwork of coral outcrops and sand.
Protected from the powerful waves of the surrounding ocean,
all kinds of animals take refuge in these calm, shallow waters.