In 1995, divers explored the Japanese coast and discovered a strange pattern on the seafloor. Upon further inspection, more of these circles appeared. They looked like geometric designs, similar to crop circles. For over a decade, no one knew who was the artist behind these etchings. But to everyone’s surprise, the creators of these 7-foot-wide (2 meters) patterns were 5-inch-long (12 centimeters) pufferfish.
The Underwater Artwork of Pufferfish
These artistic endeavors are a unique mating ritual. The male pufferfish swim along the seafloor while flapping their fins to create these intricate designs. They also add fragments of shells to decorate the edges of the circle and gather fine sand particles to give their formation a distinct color and look. It can take about 7 to 9 days for the pufferfish to complete their “crop circles.”
Then the females come to inspect the artwork. If they are impressed, they reproduce with the males, although scientists don’t yet understand what elements of these circles are deemed more attractive than others. The mating involves the females laying her eggs in the center of the circles then the males fertilizing them. The mother takes off and the father stays for about a week, perhaps to protect the eggs until they hatch. Because of the shape and ridges of the pattern, the water flow over the eggs is slowed by almost 25%.
The male pufferfish don’t upkeep their circles. Instead, the underwater currents sweep them away fairly quickly. So the pufferfish go to an area with fresh fine sentiment and build another pattern there come mating season. A study about this behavior was published in July 2013.  Alex Jordan, a researcher at the University of Texas at Austin who wasn’t involved in this study, theorized that the small fish create such large circles to compensate for some biological reason “like poor visibility at depth, or distance between individuals that means males have to make large nests to be found by females.” 
Read: Pink Meanies, A New Species Of Jellyfish With 70-Foot Tentacles, Washing Up In Florida And Alabama
“Crop Circles” Found Near Australia
Research into this species continued when scientists found similar crop circles on Australia’s North West Shelf. Marine ecologist at the University of Western Australia Todd Bond immediately knew what they were and he and his colleagues went on to found almost two dozen more designs. They reported their findings in the November 2020 Journal of Fish Biology. Although they didn’t witness the pufferfish in action, the nests are identical to the ones found in Japan. They did catch footage of a little pufferfish leaving the formation. 
A major difference between these two areas is that Australia’s circles appear on seafloors much deeper than those near Japan. This is strange since Australian pufferfish tend to live in more shallow water. “The 22 circle structures observed in north-west Australia were most likely produced by a male Torquigener sp. [pufferfish] for use as a nest,” reads the study. “Similarities in structure morphology, notably the size, number of ridges and presence of an intricate central circle with two outer rings, make them comparable to those found in Japanese waters.
“However, the circles discovered in Australia are more than 5500 km away from those in Japan and in significantly deeper water. It is unknown which species of Torquigener is responsible for producing the complex structures.” So although the culprits may be the same white-spotted pufferfish found by Japan, Australia’s seafloor may be housing a different species with similar habits — perhaps one entirely new to science. 
More About White-Spotted Pufferfish
White-spotted pufferfish live in coastal reefs, lakes, and estuaries, although the young ones prefer more shallow waters with sea grass. Their diet includes anemones, sponges, algae, coral, sea stars, and mollusks. Although their mating habits are fascinating, pufferfish are most famous for their unique defense system.
Pufferfish skin, gonads, and viscera contain a toxin produced by bacteria found in their food. This toxin, called tetrodotoxin, is dangerous and potentially fatal for humans. This poison is even more lethal if it’s injected while the fish is “puffed out”. This defense mechanism is how pufferfish get their quirky name. To do this, they swallow water to inflate themselves. Their stomach and skin stretch until they look larger, more intimidating, and much harder for a predator to consume. But in this “puffed out” mode, these fish can’t swim or steer very much. So if you see a puffer while exploring a reef, don’t try to scare it away; keep a safe distance.
Keep Reading: Extremely Rare Encounter With Deep Sea Oarfish Filmed by Divers of Coast of Taiwan
- “Role of Huge Geometric Circular Structures in the Reproduction of a Marine Pufferfish.” Nature. Hiroshi Kawase. July 1, 2013
- “Pufferfish Love Explains Mysterious Underwater Circles.” Live Science. Douglas Main. October 2, 2013
- “Pufferfish may be carving mysterious ‘crop circles’ near Australia.” Science News. Jake Buehler. October 13, 2020
- “Mystery pufferfish create elaborate circular nests at mesophotic depths in Australia.” Journal of Fish Biology. Todd Bond. August 21, 2020
- “Species Encyclopedia: White-spotted Puffer, Arothron hispidus.” Aquarium La Rochelle.