In 1995, Divers Spotted A “Mystery Circle” – It Was One Of Nature’s Greatest Creations

We didn’t quite appreciate the magnitude of their mating rituals until scientists teamed up with divers to uncover the origin of “mystery circles” popping up on the seabed. Their creations speak for themselves when seen from an aerial view, but the finer details – and more specifically, fine sand – make you realize quite how much work goes into wooing a mate for male pufferfish.

What are these “mystery circles”?

In 1995, divers discovered a curious structure off Amami Ōshima Island in Japan. There on the seabed they spotted a giant circle seemingly carved into the sand. Nobody knew where it came from, or if it was the work of a human, some other animal, or an environmental process. They came to be known as mystery circles.

A team of scientists went to investigate, and in 2011 they became the first to identify what was behind it all: a small pufferfish that somehow is able to create a structure 16 times its size using all the power it can muster in its modest 12-centimeter (4.7-inch) body. The species, Torquigener albomaculosus, was only officially described back in 2015.

Pufferfish crop circles

The scientists set up two observation areas on the sandy seabed off Seisui beach and Katetsu beach. They were able to capture the reproductive practices of males from preparation all the way through to egg care, adding up to 10 reproductive events in total.

Each one began with the creation of a circular structure on the seabed and took between seven and nine days to complete. Using their pectoral, anal, and caudal fins, they would zoom around in a circle before adding lines that went from the outside to the inside, forming radially aligned peaks and valleys.

Amazingly, they added even more refined details by stopping fin-flapping every now and then, meaning the valleys weren’t the same all the way along. Those valleys and peaks became more defined the longer they worked, and the way they swam stirred up the sand, bringing the finest particles towards the center.

The scientists also noticed that the males were decorating the peaks of their creations with shells and coral fragments, taking care to remove anything that was in the center of their construction. It was only when these final tweaks were being made that females would start to visit their creations.

Pufferfish mating rituals

When a female approached the structure, the males would swim around to stir up the fine sand in the center. If the female entered, he would then retreat before rushing towards her repeatedly. If she was suitably impressed and mating ensued, the eggs would be laid in the central part of their creation, that had now become a nest.

“In the pufferfish species investigated in this study, the nest exhibited three unusual characteristics that have never been reported in fish,” wrote the study authors. “First, radially aligned peaks and valleys were created outside the nest site; second, the peaks were decorated with shell fragments; and third, fine sand particles were gathered in the nest site to create an irregular pattern. All three characteristics were completed and maintained before mating, when females visited the nest site and they collapsed thereafter.”

A taste for the finer sand

The circular structures are beautiful to the human eye, but to a female pufferfish, it may be what sits in the middle that really wins them over. The behavior of the male and the shape of his creation all contribute towards fine sand particles moving to the center of the circle where, if he’s lucky, a female will lay her eggs.

The significance of that fine sand, the scientists say, could be why it is they don’t use the same nest twice. After this herculean effort, spending over a week to shift sand and gather shells, the males abandon the nest and build a new one. 

They might return to the same site, but the actual structures are never used twice, and they believe that may be because a single reproductive event consumes a large proportion of the available fine sand. So, he must pack his paintbrushes and start anew.

Creating incredible structures to win over a mate is something we see across multiple fish species, as well as other animal groups such as birds and mammals, including us humans. However, as the researchers wrote, these pufferfish crop circles are unique as fish artworks go, and they truly have to be one of nature’s greatest creations.