By KAZUAKI KANDA/ Staff Writer
June 12, 2025 at 18:27 JST
SETOUCHI, Kagoshima Prefecture—Mysterious geometric patterns dubbed “underwater crop circles” are appearing on the seafloor as the spawning season of a local pufferfish begins off the southern coast of Amami-Oshima island.
The intricate, circular formations are created by male Amami night-sky pufferfish as part of their elaborate courtship ritual.
On June 7, Katsuki Oki, head of the Amami Marine Life Research Association, captured rare footage of this unusual behavior in the Oshima Strait at a depth of 29 meters.
According to Oki, a male pufferfish had completed the artful sand structure the day before.
During filming, a female entered the center of the circle to spawn.
In a brief but intimate moment, the male pressed close to the female, appearing to nip at her as he released sperm to fertilize the eggs.
The male will guard the eggs, laid in the central basin of the circle, until they hatch about a week later.
“The fish is a charming and symbolic species that represents the marine biodiversity of the island,” said Oki. “Its unique spawning behavior is captivating and beloved. It’s something we should all strive to protect.”
A fully grown Amami night-sky pufferfish measures about 15 centimeters in length. The species gets its name from the star-like speckles on its back.
The fish are found primarily around Amami-Oshima island and are also believed to inhabit the nearby waters off Okinawa Prefecture.
However, the elaborate geometric spawning nests have only been confirmed off Amami-Oshima.
Local tour operators offer diving tours to observe these remarkable underwater structures during the spawning season, which runs from April to July.
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An intricate underwater spawning nest built by a male Amami night-sky pufferfish off Amami-Oshima island, Kagoshima Prefecture (Provided by Katsuki Oki)
