This isn't a starfish—it's a rare sand found only in Japan

At first glance, the beaches of Taketomi, Hatoma, and Iriomote Islands seem like any other. But take a closer look, you'll see something very special—the millimeter-sized pieces of sand are perfectly star-shaped.

According to Japanese myth, these grains of ‘star sand’ are descendants of the North Star and the Southern Cross that fell from the sky and landed in the sea of Okinawa. The god of the sea sent a serpent to kill them, and now their tiny skeletons are scattered across nearby shorelines.

Scientists agree with one part of this myth. These little stars are the skeletal remains of once-living creatures.

What is star sand?

Most sand is made of rocks, minerals, and coral worn down into tiny pieces over many years. But each grain of star sand once housed a tiny single-celled marine organism.

“Star sand is an empty shell of unicellular microorganisms called Foraminifera,” ‪Kazuhiko Fujita, a professor at the University of the Ryukyus, says in an email. “It looks like a star in a cartoon… It has a rounded body with five or more spines like a sea star.”

Living star sands are generally found on the crests (the highest point) and flats (shallow, sheltered side) of offshore reefs. “After death they are transported to the shore, but become rounded without spines,” says Fujita.

 Over time, their skeletons accumulate on beaches, forming the distinct five-pointed sand.

“When you're looking at a handful of star sand, you see lots of these little stars,” says Mark Wilson, invertebrate paleontologist at the College of Wooster in Ohio. “Each one is one of these little individuals.”

 Found on the beaches of Okinawa, Japan, and across the south and west Pacific, this special sand is very rare. “In Okinawa, some islands like Taketomi, Hatoma and Iriomote are famous for finding star sand,” says Fujita, “but it is difficult to find in other islands.”

Photosynthetic stars

These tiny organisms have a special companion. “They have these little photosynthetic algal cells, called diatoms, that live inside the skeleton,” says Wilson.

The diatoms take in light and produce carbohydrates and oxygen.

“So the individual bit of star sand—the individual skeleton—is actually a little community of the one big cell, the foraminifera, and all these tiny other cells that live within it,” he says. 

The animal's star-shaped body has tiny threads that help channel light to the diatoms so they can make food. The star has little projections radiating out from the center of the grain of sand, like the spokes of a wheel. “If you look at the tip of the projections, it's almost clear. Light shines through these channels inside the cell, and the diatoms live in those channels,” says Wilson. “It's like fiber optics on the inside.” 

Star sand is sometimes also called living sand because they add to the sand on the beach. “As their population grows, the number of those skeletons grows, so the sand itself grows,” says Wilson. This could mean “they may play some role in protecting these little islands, essentially adding material to the shores of the island.”