Parrotfish Poop Makes Up 70% of Tropical Beach Sand

Parrotfish poop is responsible for as much as 70 percent of the sand on some of the world's favorite white sand beaches, and the way they make it is stranger than it sounds.

The giveaway is the fish's beak, which looks almost exactly like a bird's and is where the parrotfish gets its name. That beak is made up of more than a thousand teeth packed tightly together, fused into a structure strong enough to bite straight through rock and coral. Parrotfish use it to scrape and crunch on reefs all day, breaking off chunks of coral and rock as they feed.

None of that coral is actually food. Once it's swallowed, the fish grinds it down further inside its gut, absorbs the organic material it can use, and pushes everything else back out. What comes out the other end is fine, pure white sand, not rock that simply wore down over time, but sand that has passed through a fish's digestive system.

A single parrotfish can produce somewhere around 300 to 400 kilograms of sand a year, and multiplied across a reef full of them, that adds up fast. Researchers who study how beaches form point to parrotfish as one of the main reasons certain tropical beaches stay so white, since coral sand keeps getting replenished as fast as waves wash it away. On top of that, all that grazing keeps reefs from being smothered by algae, which is part of why marine biologists consider parrotfish one of the more important fish on a reef, not just a sand factory with fins.

So the next time you're lying on a soft white beach or building a sandcastle, that "sand" probably isn't rock that eroded over thousands of years. It's mostly parrotfish poop, ground down and pushed out one bite of coral at a time. Technically, you're not building a sandcastle. You're building a poop castle.