How mating pufferfish created one of the ocean’s greatest mysteries

Mysterious 'crop circles' found on the seabed have been discovered to be a nest built by the most unlikely of creatures.

Several years ago, in the subtropical waters of southern Japan, divers spotted large geometric shapes sculpted in the sandy seabed. The circles were roughly two metres (6ft) across and formed of two concentric rings with spokes radiating from the centre.

They were an aquatic version of unexplained crop circles. No one could work out what, or who, had made these mysterious shapes.

The culprit? A small pufferfish

Then, a team of scientists finally caught sight of the seabed artists in action. They saw a small, male pufferfish (from the Torquigener genus), around 12cm (4in) long, darting across the sand, shimmying his fins and creating patterns in the sand. After that, more sand-sculpting puffers were spotted drawing circles on the seabed, each one performing a similar series of steps. 

How do they create these works of art?

First, a male traces basic circular shapes, then embellishes them with ridges by swimming inwards at different angles. Next, he fills in the circle with doodles of random squiggly lines.

For a finishing touch, the male gathers up pieces of dead coral and seashells to decorate his seabed creation. The whole process takes at least a week.

Underwater crop circles are functional, too

The purpose of these shapes becomes clear when female pufferfish show up and inspect a male’s artworks. As it turns out, these shapes are nests.

The circular design seems to channel fresh water towards the middle of the nest, so no matter which way the current is flowing, fresh oxygen-rich water is swept into the central spawning area, creating ideal conditions for egg development.

Male pufferfish are very hard-working

If a female pufferfish likes the look of a nest, she’ll lay her eggs in the middle then swim off, leaving the hard-working male to hang around for another six days, guarding the nest and the growing eggs, while his artwork gradually crumbles and gets swept away. Every time he wants to attract another mate, he has to build a new nest. 

Asked by: Jermaine Reyes, via email

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