A male pufferfish digging away, by K. Ito
We humans tend to think of ourselves as rather unique creatures. But even as we stand in our crystal skyscrapers, flexing our muscles out floor-to-ceiling windows at the nature we think we've conquered, we forget that we're not the only architects in the animal kingdom.
Bowerbirds are some of the most acclaimed animal builders, but one surprisingly skilled group are pufferfishes, which build large circular structures on the ocean floor. Naturally, these underwater crop circles aren't built just to freak out divers, but to attract mates. They're geometric marvels. I mean, just look at these things:
The figure above comes from a paper published in July, and shows the progression of the nest of a single male pufferfish. (A through C show it being constructed, and D shows it after the fish stopped maintaining it.) According to the paper, "Role of Huge Geometric Circular Structures in the Reproduction of a Marine Pufferfish," the nests built by fish in the Torquigener genus can reach a diameter of around two meters.
A six foot underwater circle with near perfect patterns is impressive enough, but then there's this: Torquigener species range from roughly 10 to 20 centimeters in length. (The authors, Hiroshi Kawase, Yoji Okata, and Kimiaki Ito, note that the first fish they observed building a nest was 12 cm long.) That such small fish could dig big circles and consistent valleys—done with their fins and body—is incredible.
According to the paper, the structures were observed in Japan's subtropical waters, and took about seven to nine days to complete. That's a lot of work, and in addition to creating the radial valleys around the circle, the fish were observed to create irregular patterns of fine sand particles. But it's all worthwhile, as the elaborate nests and use of fine sand were both found to influence mate choice. Although it's not totally clear what characteristics females judge the nests on, it appears that collections of fine sand in the center nest region is important.
This video plays up the "are they aliens" angle so well.
Female pufferfish are apparently so interested in this fine sand that males abandon their nests after one use. "Strangely enough, the males never reuse the nest, always constructing a new circular structure at the huge cost of construction," the author wrote. "This is because the valleys may not contain sufficient fine sand particles for multiple reproductive cycles."
The paper is one of the first studies with underwater observation of nest construction in Torquigener. The nests were first observed in the 90s by Japanese divers, and have been referred to as underwater crop circles ever since. But more impressive than underwater aliens sending messages to divers is the truth: Small fish working for days at a time build these large structures, all in the hope of attracting a mate.