The Mystery of Underwater Crop Circles, Explained

They’ve been called the crop circles of the ocean floor—seven-foot diameter patterned circles that were first spotted in 1995 off the coast of southern Japan. But their origin was an enigma, and local divers termed them “mystery circles.”

The mystery persisted until 2011 when the culprit, a male pufferfish just five inches long, was finally caught in the act. And recently scientists studied the process of how the species creates these elaborate designs in order to woo females.

Finned Diggers

The research team observed a total of 10 construction events carried out by somewhere between 4 and 8 males. (Pufferfish don’t have very memorable faces, apparently.) Males spent seven to nine days building their respective circles by repeatedly swimming in and out of the circle, using their fins to dig valleys in the sandy bottom.

Aesthetics were clearly important. The spirograph pattern was meticulously created and males were observed decorating the peaks with shell and coral fragments. But the design had a practical purpose as well: the male’s swimming pattens stirred up fine sand particles and pushed them toward the middle of the circle, which served as the actual nest.

This was the part of the circle where he entertained lady callers. When a female pufferfish approached the circle, the male stirred up the sand in the middle and darted back and forth through it. If she judged him a suitable mate, she would lay her eggs in the sandy central zone, the researchers reported last month in Scientific Reports.

Build and Rebuild

Scientists say it’s likely that the quality of the circle helps determine a female’s mate choice, though they have yet to demonstrate how. But once mating is completed, the male ceases his upkeep of the circle, and after the eggs hatch, he abandons the nest altogether.

But after all that effort, you may ask, why not just reuse his earlier circle? The authors speculate that the male’s forceful wooing depletes the area of its fine sand particles, which are necessary for the next round of egg-rearing. And then it’s back to the drawing board for these amorous artists.