White Sand

On a reef island in the Maldives, more than 85 per cent of the new sand produced each year on the surrounding reef flat comes from parrotfish.¹

The fish bite into coral, digest the edible part and pass the rest as sand.²

Parrotfish are a family of about 90 species that live in shallow tropical and subtropical seas.³ Most graze on algae, scraping it from living and dead coral with fused teeth, and they swallow fragments of the coral as they feed.³ A set of plates in the throat called the pharyngeal mill grinds those fragments down, and they leave the gut as fine white sediment.³

The most detailed measurements come from Vakkaru, a sand island inside a Maldivian atoll.¹ A team led by Chris Perry, Professor of Geography at the University of Exeter, published a budget of the island's sediment in 2015.^{1, 2} The surrounding reef produces about 685,000 kilograms of sediment a year, and about three quarters of it comes from the narrow outer reef flat, which covers just 21 per cent of the platform.¹ There, parrotfish generate more than 85 per cent of the new sand-grade material, about 5.7 kilograms for each square metre every year.¹ The alga Halimeda contributes a further 10 per cent, most of it as coarser gravel.¹

Elsewhere the figures are broader estimates.³ The University of Hawaiʻi estimates that up to 70 per cent of the sand on white beaches in the Caribbean and Hawaiʻi came from parrotfish, and that a large adult can produce about a ton of sand a year.³ This matters particularly in Hawaiʻi, because little sand arrives from the land and nearly all of it is made by living things.³

Reef islands are among the landforms most exposed to rising sea levels.² Perry and his colleagues concluded that Vakkaru depends on a narrow strip of productive reef and on healthy parrotfish populations to keep its sand supply going.^{1, 2}

A beach on such an island is not simply there. It is renewed, a few kilograms a square metre at a time, by the animals feeding on the reef around it.