What creates mysterious circles on the seafloor? No, it’s not aliens of the deep—it’s actually pufferfish hoping to snag a mate, a new study says.

Divers first noticed the 6.5-foot-wide (2-meter-wide) circular structures near Japan‘s Amami-Oshima Island about 20 years ago. But no one knew how these so-called mystery circles were constructed—or what was creating them—until now.

The circles, scientists say, are actually nests created by male pufferfish, which spend about ten days carefully constructing and decorating the structures to woo females. What’s more, this industrious pufferfish is thought to be a new species in the Torquigener genus, according to the study, published July 1 in the journal Scientific Reports.

Nesting Instinct

The male fish, which measures less than 5 inches (13 centimeters) long, first uses his body to create peaks and valleys in the sandy bottom around a central circle of smooth sand. He accomplishes this feat by swimming in toward the center of the circle in a straight line and then back around the center in a circular motion. (Watch a pufferfish video.)

Before the female fish arrive to inspect his handiwork, the male forms irregular patterns in the fine sand particles of the central circle. He also decorates the peaks of the outer portion with shell and coral fragments.

When a potential female partner arrives on the scene, the male stirs up the fine sand in the nest’s inner circle. If she deems the nest, and the male who built it, satisfactory, she lays her eggs in the center of the nest and leaves.

The scientists aren’t sure exactly what the females are looking for when they judge a male’s nest. It could be the central patterns made of fine sand, the decorations on the outside, or the nest’s size or symmetry. (Also see “Help Name This Mystery Fish.”)

But they do know that because the nest-making process is so time-consuming, a larger nest could indicate a stronger or more fit male—both desirable traits to females.

Dad Duties

Once the female splits, though, it’s the male who does the parental chores: He remains in the nest until the eggs hatch six days later.

Afterward, the male looks for a nearby spot to start the nest-making process all over again, which is a mystery of its own: Why spend all that time and energy building a brand-new nest? The scientists think it has something to do with the fine sand particles that make up the smooth center of the circles. (Read “Africa’s Mysterious ‘Fairy Circles’ Explained.”)

The males use up all the fine sand within the radius of their nest during a single spawning cycle. To construct another nest, they have to look elsewhere for a site with more fine sand particles.

Now that’s what you call a labor of love.

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