The Netflix documentary Chasing Coral makes passing reference to an extraordinary claim: Beaches are made of parrotfish poop.

Here’s the thinking: These colorful beaked creatures of the reefs eat coral and grind up their calcium carbonate skeletons, which they then they poop it out as sand.

This is sort of a half truth. While parrotfish-processed sand is a major component of some of the world’s beaches, it is not the case everywhere. If you want to know if a particular beach has a shitty past, you’ll need to first investigate what its sand is made of.

Sand is not defined by its composition but by its size; it is any mineral material coarser than gravel but finer than silt. Sand beaches are made by erosional processes that wear down rocks over eons. The white, powdery sand that made Florida’s beaches famous is not parrotfish crap — it is the end process of millions of years of the weathering of quartz rock, until all that is left is tiny crystals of pure silicon dioxide, all of the same size and shape.

Beaches can be made from all sort of materials — anything that is rock can be made into sand. Where parrotfish involved, though, is when it comes to coral sands. If your beach is in a coastal ecosystem where coral reefs dominate, then your sand might have been through the innards of a fish before it became your castle.

The contribution of parrotfish poo to beaches will vary from place to place, but one study found that 85 percent of the sand produced at Vakkaru Island in the Maldives was pooped out by a fish.

Most parrotfish are herbivores, and they play a vital role in reef ecosystems. They eat the algae that compete with coral for territory, and by cutting back the green stuff they allow coral to stand its ground. They also eat algae that grow directly on coral, which would smoother it if left to grow unchecked. One Carribean study found that protecting parrotfish and other grazers is the most important thing we can do to promote coral reef health.

These animals also ingest bits of coral skeleton, made of calcium carbonate, that is stuck to the algae. This is ground down by special teeth in the fish’s throat, and out it goes the other end.

More significant, though, is the contribution of predatory parrotfish. Some species go after coral polyps themselves, digesting the fleshy bits as food and pooping out the hard stuff. A single parrotfish can produce 200 pounds of coral sand each year in this way.

While it may not seem like eating coral would help reefs, these parrotfish actually play an important role in building reef ecosystems. The sand they poop out acts like a filler in the physical structure of the reef, supporting the coral skeletons that make its pillars.

So yes, some beaches and indeed whole islands and countries are made mostly of parrotfish poop. If your beach is near a healthy population of coral-eating parrotfish, it just might be among them.