An island in the Maldives is made of parrotfish poop

Vakkaru Island in the Maldives is little more than a spit of sand rising above the ocean, topped with a bit of vegetation and surrounded by coral reefs. Vakkaru is a reef island, which means that it’s made up of sediment produced on those reefs. There are around 1,200 islands like it in the Maldives, clustered in groups along the region’s numerous atolls. But where does the sediment come from? Mostly fish, a new study finds.

Specifically, fish feces.

Many species of parrotfish dine on coral polyps. But instead of delicately pulling the polyp out from its hard coral exterior (like how you might eat a snail), the fish just chomp on the stony reef, ingesting polyp and hard coral together. Inside the parrotfish, the polyp gets digested and the stony coral excreted. Parrotfish create so much sediment that have created entire Hawaiian beaches.

Chris Perry of the University of Exeter in England and colleagues wanted to know where the sediment supplying Vakkaru Island originated. So the team surveyed the waters off the island, mapping the species that are capable of producing sediment and then tallying up how much sediment each would be capable of producing over a year.

About 685,000 kilograms of sediment is produced annually in the waters off Vakkaru, the researchers report April 27 in  Geology . Based on the size of the sediment grains, the source wasn’t simply coral rubble. Some species of sponges  are capable of boring into coral reefs and creating sediment, but the survey found that these types of sponges were rare. And those that were present create silt-sized sediment, which was likewise rare on the island. And grazing sea urchins can produce sediment, but the grains weren’t the right size.

About 10 percent of the sediment, the researchers found, could be traced to Halimeda algae. These algae have calcareous plates that, when they break off, fracture and eventually become tiny bits of sand. But the vast majority — more than 85 percent — of the sediment came from parrotfish. And two species of “excavator” parrotfish, Chlorurus sordidus and C. strongylocephalus, were the biggest sources. The fish take in the coral and poop out sediment, and those tiny grains then get transported to the island around the time when the seasonal monsoon wind shifts.

“The maintenance of the healthy parrotfish populations that characterize these Maldives reefs … is thus critical to sustaining contemporary sediment generation regimes, and appears necessary for island building and maintenance at this site,” the researchers write. And with sea level rise eating away at islands like these, the fish are more important than ever.