The Culprit Behind The Mystery Circles: Bombs? Craters? Love?

There is a large difference between scientific discovery and scientific explanation. After something is discovered, the public sometimes waits decades before it is fully explained to them, and this process was no different for a series of crop-circle-esque formations found on the seafloor near the southern part of Amami-Oshima Island in 1995 Japan, and later found off the coast of Australia. These circles were first discovered at depths greater than 30 meters beneath the surface in shallow, sandy areas of the Sea of Japan. With widths of around two meters, they seemed far too large to be made by any kind of accidental sea creature or other known phenomenon.

These structures are made up of concentric circles formed by sand on the seafloor. The smaller, center circle is elevated slightly above the area between the two, and that divot is covered with lines radiating from the inner to the outer border. The outer, larger circle is also raised slightly above the seafloor; slightly below, channels between the outward lines for the second circle are depressed.

Scientists didn’t publish a true explanation for these circles until 2011, when they observed a new species of pufferfish—the white-spotted pufferfish (Torquigener albomaculosus)—actively digging and flailing its fins to create a new structure. After a series of repeated observations in 2011-2012, scientists were able to determine that these structures were used both as nests and as displays of courtship.

Many members of the animal kingdom use elaborate nesting and courtship methods to get mates. Some fantastic examples of this are birds of paradise, which are a family of tropical birds native to Australia and Southeast Asia. For these species, the determining factor for continuing their lineage is their dance skills. Each species has a unique mating show, but many use brightly colored feathers, song, and dance to impress mates. Similarly, male bowerbirds impress their mates by building stick structures that can be up to a meter high. These feats demonstrate the health, fitness, and vigor of the bachelor to the female so that she can choose the strongest one to reproduce with. Each species has its own unique technique because each needs to show off its own specialized traits, but many techniques involve displays of strength or endurance.

Though this courtship is less common in fish, the white-spotted pufferfish clearly displays such characteristics. Given that she chooses him, the male shuffles around the sand to both impress the female and for practical purposes. Females can choose their mates based on the size and craftsmanship of these nests. This is evolutionarily advantageous to the species because larger, stronger males will build larger, stronger nests, so the species will tend to evolve towards a more evolutionarily optimal form.

Once the female chooses her mate, the rings will serve another purpose as a nest for the eggs. The female will lay her eggs in the center on the elevated platform, and the male will fertilize them. The geometric form of the nest will funnel highly oxygenated water—which is necessary for fish to breathe just like for humans—into the eggs through the grooves on the outside, and the fine sand placed at the center will provide a good environment for them to develop. This incredibly complex structure is likely the result of hundreds of thousands of years of evolution and selective survival, where only the fish that built effective nests were able to survive and reproduce.

While they are not the first example of finding a record of the animal before the culprit, the white-spotted pufferfish is a fantastic example of the beauty and wonder of nature, as well as a reminder that there is still so much to learn about the world around us, especially in the sea. Due to recent technological advancements, there are far more opportunities to explore and understand the intricacies of both sea and land creatures.