Mystery of underwater circles solved

These 'mystery circles', which appear on the seabed off the coast of Amami-Oshima island in subtropical Japan, have been baffling divers for decades - but now researchers have found out not only what makes them, but why.

A team led by Hiroshi Kawase of the Natural History Museum and Institute Chiba discovered the culprit was a small male puffer fish measuring around 12cm/4.75" in length, which spent a week or more constructing these geometric circular structures 20-30m below the surface in order to attract a mate.

The puffer is thought to be a new species of the genus Torquigener.

The male forms a basic circular shape at first, then meticulously creates peaks and valleys in the floor using his pectoral, anal and caudal fins. The central area of the circle at this stage is kept flat.

Just before spawning, he uses his anal fin to build an irregular pattern in the centre of the circle with fine sand stirred up in the creation of the peaks. He then gets really arty, decorating the peaks of the circle with shell and coral fragments. Only when this is complete will a female be enticed in for spawning to take place, with the eggs being released in the central zone.

After spawning the male stays within the circular structure for around six days to care for the eggs, although he doesn't maintain the area during this period and the structure eventually begins to disappear, flattening out due to the water currents and being covered by coarser sand particles.

Once the eggs hatch, the male leaves the nest site, before returning to the area to start the cycle again. However, the team found that the male never returns to an old nest site, instead constructing a new structure at a different site. The construction of each nest site was found to take 7-9 days.

For more information, see the paper: Kawase, Okata Y and Ito K. Role of Huge Geometric Circular Structures in the Reproduction of a Marine Pufferfish. (2013). Scientific Reports 3, doi:10.1038/srep02106.

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