Japan In Focus

Kagoshima: ‘Mystery Circle’ Made by Puffer Fish on Seabed; Males Stir Up Sand to Make Spawning Areas

Courtesy of Katsuki Oki
A spawning bed and Amami night-sky puffer fish that created it are seen off the coast of Amami Oshima Island in Kagoshima Prefecture on June 7.

The Yomiuri Shimbun

15:15 JST, July 5, 2025

AMAMI, Kagoshima — A circular geometric pattern was found this year on the seabed off the course of Kagoshima Prefecture’s Amami Oshima Island. The 2-meter diameter pattern is a puffer fish spawning bed.

The spawning ground was created by a species known as Amami night-sky puffer fish, which themselves measure about 10 to 15 centimeters in length. The pattern, referred to as the “seabed mystery circle,” was found at a depth of 29 meters by Katsuki Oki, the head of the Amami Marine Life Research Association, among other parties, on June 6. The association conducts research on the ecology of marine life in the waters near the island.

According to the association, male puffer fish create these spawning beds by stirring up the sand on the seabed using their fins to draw female fish. On June 7, a female was observed laying eggs at the center of the “mystery circle.”

Courtesy of Katsuki Oki
Amami night-sky puffer fish are seen off the coast of Amami Oshima Island in Kagoshima Prefecture.

The male that created the bed would then release sperm there and guard the eggs for about a week until they hatch. This is a common sight each year during the species’ spring-to-summer breeding season.

“Their breeding behavior cannot be observed unless their environment is in good order, with sandy, mostly flat seabed with small stones,” Oki said. “We hope this rare behavior can be carefully observed.”