Pufferfish and the High Price of Love: Underwater Crop Circles?


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This male pufferfish has perfected the art of attracting a female! Image courtesy of Dr Kawase

Is it the crops circles again? No, this time it’s the seabed circles!

In 1995, divers spotted for the first time strange geometrical structures measuring about 2 meters in diameter on the seabed near southern Amami-Oshima Island in subtropical Japan.

For 16 years nobody knew who or what could build these structures. But in 2011, in a twist similar to the crop circles, a team of researchers from the Coastal Branch of Natural History Museum and Institute in Japan finally uncovered a perfectly simple and innocent explanation.

Pufferfish Architecture: Strange Structures Found on Seabed

The ‘culprit’ for these large enigmatic structures is a tiny (but determined) silvery male pufferfish in search of love.

Diving with a video camera in hand, marine biologist Dr Hiroshi Kawase and his team managed to film a total of 10 reproductive cycles with different males, from preparing the nest to looking after the eggs.

The study was published in the July edition of Nature Scientific Reports.

Building Elaborate Nests: Nothing is Too Much for a Fish Looking For Love

In what must feel like a herculean task, building the nest takes about 7 to 9 days and follows a strict plan, as if male pufferfish have detailed construction drawings. During the first few days, males use their fins to set up the basic shape of the nest. With ingenious and meticulous finning, they create an intricate pattern of ‘ups’ and ‘downs’, forming valleys and peaks on the outside of the nest. The central zone, in contrast, needs to be flat. For this, males swim frantically backwards and forwards in this area, to stir up the sand.

Not happy with the result, males often return to the outside area to perfect every single peak and valley, ensuring that they’re clearly defined and identical. The fact that they spend a few days doing this shows how important it is.

We’re not sure how the female chooses a male, explains Dr Kawase, but “it appears reasonable to assume that females evaluate nest characteristics and that these play an essential role in female choice.”

On the last day, males get in touch with their ‘feminine side’ and find broken shells and bits of coral to decorate the peaks. Using fine sand stirred while doing the peaks, they also create an irregular pattern in the middle zone, which they hope will please any visiting female.

Female Pufferfish: Here Come the Girls

Finally, girls are allowed to visit. Like a ‘little boy with a new toy’, males are keen to show off their nest. While the female inspects the centre, the male swims up and down waiting anxiously for her verdict. It’s a tense few seconds, but if it’s a yes, mating can occur and the female lays the eggs in the central zone, protect by the surrounding peaks.

Dedicated Fathers, but not DIY Experts

The males’ input is not over, though. “Males remain in the circular structure for 6 days to care for the eggs”, says Dr Kawase, but “they don’t do any maintenance during this period”. As a result the nest gradually disintegrates, but, like devoted fathers, males stay with the eggs until hatching.

Afterwards they briefly disappear, but soon return to the same area for another cycle. They always pick a different location and start afresh with a new nest. This may seem odd at first, but there’s a good reason. “Males construct new structures,” explains Dr Kawase, “because a single reproductive cycle consumes a large proportion of the available fine sand particles in the area. Males must construct a new circular structure at a separate site where sufficient fine sand particles are available.”

Seabed Construction: High Price for Love

There are many different species of fish known for their seabed constructions, but none as elaborate as this. It’s a high price for love, but one that pufferfish seem willing to pay!


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. Accessed August 6, 2013.

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