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Find out how parrotfish, zooxanthellae, calcium carbonate (aragonite), and poop combine to make the white sandy beach sunbathers love.

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Let’s say you wanted to make yourself a white, sandy beach.

Y’know, like the ones in Hawai’i or islands in the South Pacific where people take luxurious vacations and lots of Instagram photos. It turns out, the recipe for that picturesque white sand is pretty simple.

There are a couple steps involved, but just one main ingredient: poop. The first thing you’ll need is a coral reef—it’s location-specific and unfortunately there’s no store-bought alternative. Coral reefs are made up of thousands of tiny polyps, which are squishy sacs with tentacles that can sting and catch prey.

Kind of like sea anemones. These polyps are anchored to a sturdy skeleton that they make by laying down a crystallized form of calcium carbonate called aragonite, which is a bright white color. Reefs are found in parts of the ocean that are considered to be oligotrophic—which means that there aren’t that many tasty nutrients floating around, like phosphate or nitrate.

And coral polyps aren’t exactly mobile, so if they were left to their own devices, they might starve. But coral polyps aren’t alone. They get help from an algae called zooxanthellae that live symbiotically in their tissues.

These algae can photosynthesize, changing sunlight, carbon dioxide, and water into food energy, and share a large chunk of that energy with the coral. As coral polyps get more energy, they build out their skeletons and grow the reef. And these algae are also what give coral its bright colors.

When you have a thriving coral reef, other undersea plants and animals move in and begin to form a whole ecosystem. And to get your white sandy beach, you need some parrotfish. Or more specifically... their poop.

Parrotfish get their name for their bright colours and their weird, bird-like beaks. These beaks are actually made of fused teeth called dental plates, which help them graze for their main food source: algae. Parrotfish spend their days scraping away at coral reefs, eating polyps for the zooxanthellae inside, and any other algae they can find.

They’re not super careful or picky, so they end up swallowing mouthfuls of calcium carbonate, too… which they can’t digest for nutrients or energy. But the parrotfish have a secret weapon—their throats are hiding another set of teeth called pharyngeal jaws that can grind up the coral skeletons. Like, the xenomorph from Alien… but in real life.

This makes it easier for the chunks to travel through their digestive system. And they get rid of this calcium carbonate in streams of the purest, whitest tropical sand. In fact, according to some studies, one parrotfish can poop out around 300 kilograms of sand or more in a single year!

After some ocean currents stir everything around, you have yourself a pristine white sand beach. Perfect for strolling, sunbathing, and building sandcastles… as long as you don’t mind fish poop. Thanks for watching this episode of SciShow, brought to you by our patrons on Patreon.

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