The parrotfish beak is formed from fused incisors that scrape or bite coral. Behind it sits the pharyngeal mill, a second set of jaws in the throat lined with rows of hard teeth that pulverise the swallowed skeleton so gut enzymes can reach the algae and cyanobacteria living inside it. The indigestible calcium carbonate passes straight through.
Published estimates for the green humphead run to roughly five tonnes of reef carbonate processed per adult fish per year, and reef-wide surveys attribute a substantial share of the white sand on tropical beaches to parrotfish digestion. Smaller species contribute proportionally less, but there are far more of them.
The headline figure is easy to overstate. Estimates vary widely with fish size, species, reef type and the method used, and the often-quoted single number for all parrotfish conflates species that differ by two orders of magnitude in bite volume. What is well supported is the direction: parrotfish are among the dominant producers of reef sediment.