Almost all the soil of coral islands like Hawaii, and the sand of their beaches, must come from the reefs that surround them - there isn’t any other likely source. So how does that happen? A group of scientists set out to answer this question and published a research paper in the Geological Society of America’s journal in 2015.
They studied previously-collected data from Vakkaru island in the Maldives in an attempt to work out what habitats the sediments were created in and what kind of animals played the biggest part in this process.
They estimated that animals created nearly 700 tonnes of sediment each year, mostly on the narrow outer reef. It turned out that the biggest producers of sediment by far were Parrotfish.
Parrotfish eat seaweed that grows on coral. Some species chew several millimetres into the coral to reach the deeper-growing algae. The beak-like array of fused teeth that gives the parrotfish its name is an adaptation for rasping coral. Tiny scrapings of coral that get eaten along with the algae pass straight through as poo and fall to the bottom, creating sediment.
The Parrotfish Song
Besides creating sand, parrotfish are essential for the health of reefs in another way - their constant grazing prevents the coral being smothered by seaweed. Unfortunately, parrotfish populations are under severe pressure from over-fishing.