Parrotfish Build Islands With Their Poop

Parrotfish munching on algae ingest coral and then eliminate the rocky substrate, creating island-building grade sediment in places like the Maldives. Julia Rosen reports.

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The form a constellation of almost 1,200 coral reef islands in the Indian Ocean. They have stunning white sand beaches surrounded by emerald blue water. And according to a new study, they may owe their existence to . More specifically: to parrotfish poop. C.T. Perry et al, , in ]

If you’ve ever snorkeled near a coral reef, you’ve probably seen neon-colored parrotfish. Their name refers to their sharp, beak-like teeth. You may have heard them too. [Scraping sound] That’s a parrotfish literally eating the reef’s coral skeleton. It bites off tiny pieces of hard coral as it forages for algae.

“That gets taken into the fish. It’s then milled. And it passes through their intestines and it’s then excreted out the back end as clouds of sediment.” Chris Perry, a marine geoscientist at the University of Exeter in the UK.

“And that is then distributed onto the reef. And it’s a way, at least in this system and elsewhere parrotfish are abundant that you can convert coral substrate into sediment grade material.”

In 2013, Perry and colleagues went to the Maldives to find out how the reef—which grows underwater—generates sediments that pile up, forming islands that rise above the water’s surface. The team discovered that parrotfish play a critical role in this process: their waste accounts for a whopping 85% of all the sand produced on the reef.

“If you spend time on the reefs and you’re following the parrotfish around, it’s absolutely incredible how much sediment they’re producing. So when you’re got large populations of them, they can be very important in this respect.”

The new results suggest that protecting parrotfish populations is essential for maintaining the Maldives, which already face threats from rising seas.

“If you take the parrotfish out of these systems you would basically be shutting down a very significant amount of the sort of supply chain for island-building sediment.”

By the way, the beautiful white sand beaches in parts of Hawaii? Also largely parrotfish poop. Think about that next time you’re spreading out your towel in Oahu.

—Julia Rosen