Pufferfish Love Nests

Mysterious undersea circles...
Image by Yoji Okata

Male pufferfish Torquigener sp.
Image by Yoji Okata

Last year, images of these strange, undersea geometric structures first appeared on the internet. These rippling structures were discovered in 1995 by Japanese diver and photographer Yoji Okata. They are about 6 feet (2 m) across and are found off the coast of Japan in about 80 feet of water. Although initially mysterious, in 2011 it was found that these structures were built by little male pufferfish of the genus Torquigener, which themselves are no larger than about 5 inches (12 cm).

Male pufferfish at work building the nest.
The scale of the nest in comparison to the fish can be appreciated.
Image by Yoji Okata

It turns out the male pufferfish build these large nests to attract females to mate. Males going out of there way to do things to attract females is a familiar theme across animal life. Male bowerbirds are well known for building intricate nests to attract females. While nest-building is well-known among many species of fish, building the nest with these radial peaks and valleys, as well as decorating it with shells, has never been seen before.

In July, the scientific paper describing these nests and the pufferfish behavior appeared in the journal Scientific Reports, a relatively new journal published by Nature Publishing Group. But even cooler than that, it came with new pictures and videos! The appearance of the paper caused a small resurgence in social media on the discussion of the pufferfish. No one's really mentioned the videos, though, which is a shame because the paper is actually open access and so the videos are free to watch! If a picture is worth a thousand words than video is even better, right?

Male pufferfish building a nest, using his fins and body to dig valleys.
Photo by K. Ito

The only image of a pufferfish in the entire paper isn't anything new compared to the images that were already out there, but yet again demonstrates a male Torquigener hard at work building a nest. You can see how the male pufferfish uses its fins and body to dig the valleys into the sand. The male pufferfish will take seven to nine days to build the entire nest, but when attracting females is the game males across the animal kingdom do some pretty involved things.

The paper also describes the nest-building behavior more specifically, includes the steps for how the nests change over time and how the pufferfish behaves over time, and some video documentation.

Caption from paper: "Changes in the circular structure constructed by male pufferfish. (a) Early stage; (b) middle stage; (c) final stage; and (d) after spawning of the same circular structure of K1 in Figure 2. Photograph by Y. Okata on 23, 27, 29 June, and 6 July 2012, respectively."

The above image shows off the stages of nest-building. The nests start off roughly circular, and the valleys and hills get more pronounced over time (a-c). Not longer after spawning, however, the structure is left alone (d) and fades away.

Several videos are also available in the supplementary information, where you can the various male pufferfish digging behaviors. You can see how it uses its bodies and then waves its fins around, sometime sinking down into a spot and beating its fins to dig deeper pits. You can also see how the male pufferfish nicely fits in the valleys. The pufferfish keeps the center smooth with fine sand particles, and at the end of construction gives it a pattern (c). There's even a shot where it picks up stuff (which the paper says are shells) with its mouth and puts it down on the outer peaks. The last video is of the male attempting to court a female. Definitely very cool behavior to watch!

Sand particles from the nest the day before spawning and the day before hatching.

If the male is successful at courting the female, they will spawn and the eggs are laid in the central portion. Like in many fishes, the male will be the primary caretaker, and the female goes off on her own while the male hangs around for about 6 more days to care for the eggs, during which he stops maintaining the nest structure (d). Lack of maintenance of the nest also reduces the amount of fine sand particles in the center of the nest. Because of this, it's inferred that the complex nest building seems to be important for attractive females, but not so much for actually raising the young.

While the nest is already just pretty neat to look at and impressive to boot, according to the paper, the radial pattern, the central patch of fine sand, and the decoration of the nest with shells are apparently all characteristics never seen in fish nests before. Also, the peaks and valleys of the nest affect the flow of water, and allow for fine sand particles to flow into the valleys, which are then moved to the inner circle by the male. Interestingly enough, the male pufferfish don't re-use their nests. The authors hypothesize that this could be because the fine sand particles get used up, and so they have to find a new site to rebuild.

All in all, this is pretty cool behavior, but plenty of questions remain. The authors note that the specific reasons the nests are designed are unknown; why does the puffer build the nest symmetrically, or with certain numbers of peaks and valleys, certain peak heights and valley depths? How does it choose how to decorate the nest site with shells? There are further questions as well. What's the function of fine sand particles in attracting females? Do other Torquigener species build nests?

These types of discoveries also stress how little we still know about the deep. It was only two years ago when the puffers were finally observed building nests. We can be sure that other secrets remain to discover under the sea.