There are a million mysteries in the Naked City of Biology, and some of them get solved. This is one of them, taken from the Japanese culture site Spoon & Tamago. It’s described in the post “The deep sea mystery circle—a love story”

Several decades ago, a Japanese “salary man” named Yoji Ookata quit his office job to pursue his real love—underwater photography. Recently, diving 80 feet down off the island of Amami Oshima (one of the Amami islands between Japan and Taiwan), Ookata saw something that nobody had ever seen before. It was a large, radially symmetrical pattern in the sand, and looked like this (note underwater camera for scale):

The site describes this bizarre structure:

On the seabed a geometric, circular structure measuring roughly 6.5 ft in diameter had been precisely carved from sand. It consisted of multiple ridges, symmetrically jutting out from the center, and appeared to be the work of an underwater artist, carefully working with tools. For its resemblance to crop circles, Ookata dubbed his new finding a “mystery circle,” and enlisted some colleagues at NHK [a Japanese television station] to help him investigate. In a television episode that aired last week titled “The Discovery of a Century: Deep Sea Mystery Circle,” the television crew revealed their findings and the unknown artist was unmasked.



What on earth could have caused that? Was it some kind of elaborate prank, like crop circles? Not likely: this structure wouldn’t retain its integrity for long in the face of currents. But what? Try to guess before you read further.

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It’s a fish! Or, rather, a single small male pufferfish who digs the structure in the sand:

Here it is digging:



Now why on earth would a small fish go to all this trouble to do this? (Note how elaborate the sculpture is, and how symmetrical.)

If you’re an evolutionary biologist, you might have guessed: sexual selection. A male sculpts this thing to attract females for mating. And the sculpture is decorated, too!

Underwater cameras showed that the artist was a small puffer fish who, using only his flapping fin, tirelessly worked day and night to carve the circular ridges. The unlikely artist – best known in Japan as a delicacy [JAC: perhaps this is the fugu, or edible pufferfish that has a toxic liver], albeit a potentially poisonous one – even takes small shells, cracks them, and lines the inner grooves of his sculpture as if decorating his piece. Further observation revealed that this “mysterious circle” was not just there to make the ocean floor look pretty. Attracted by the grooves and ridges, female puffer fish would find their way along the dark seabed to the male puffer fish where they would mate and lay eggs in the center of the circle. In fact, the scientists observed that the more ridges the circle contained, the more likely it was that the female would mate with the male. The little sea shells weren’t just in vain either. The observers believe that they serve as vital nutrients to the eggs as they hatch, and to the newborns. What was fascinating was that the fish’s sculpture played another role. Through experiments back at their lab, the scientists showed that the grooves and ridges of the sculpture helped neutralize currents, protecting the eggs from being tossed around and potentially exposing them to predators. It was a true story of love, craftsmanship and the desire to pass on descendants.

What we have here, then, is the underwater equivalent of the bowers built by Australian bowerbirds: elaborate structures to attract females (bowers, too, are often decorated, and you can see their variety among species here). One difference between the pufferfish’s structure (perhaps readers would care to name it?) and bowers is that the latter are built by birds solely to attract females, and are places where matings occur. Eggs are not laid in the bowers, but elsewhere in regular nests. The deadbeat male absconds for good after mating.

In both cases, it seems, the more elaborate the structure, and the more decorations it bears, the more attractive it is to females. We’re not sure why this is: perhaps it’s a sign to the female of the male’s vigor (indicating either good genes or the ability to dispense lots of good paternal care), or perhaps females simply have an inherent preference for more elaborate structures (I doubt this, but that was Darwin’s theory. He thought that females had an innate aesthetic sense that males evolved to cater to). Females may also have the ability to judge whether the ridges are good enough to protect their eggs.

Evidence favoring the good-genes model is that males of both bowerbirds and pufferfish don’t tend the eggs or offspring after they’re laid, so there aren’t any “direct benefits” a male able to build nice structures can give to his offspring (except making the ridges good enough to protect eggs from currents). Beyond that, males bequeath only genes. I asked my colleague Steve Pruett-Jones—an expert on the evolution of bird behavior, who also gets up early—about bowers, and he sent this answer:

Males never see the nests of females (as far as anyone knows). Despite the ‘bower’ and all of the unusual aspects of bowerbirds, the situation in bowerbirds is exactly the same as it is in birds of paradise or any other lekking species [JAC: “leks” are behaviors in which males of a species gather together in competitive displays to attract onlooking females], whether they be grouse, manakins, hummingbirds, fish, flies, etc. The question of why females should prefer males with elaborate bowers is exactly the same as why females of lekking species should prefer the males that they do. Females only get genes, nothing more. And, you know at least indirectly the literature on lekking species. There are lots and lots of correlations between aspects of male display and mating success, but very few data on the benefit to females of making the choices that they do. It is unlikely that there is only one answer, but people now generally accept the notion of good genes (as opposed to ‘no’ benefit through runaway sexual selection). Nevertheless, figuring out what those genes are and what they do for the female’s offspring remains a challenge.

There are other explanations for female preference, too, but I won’t go into them. A firm explanation for female preference still, as it does so often, eludes our grasp.

To close, here is an elaborate bower built by the male satin bowerbird, Ptilonorhynchus violaceus. Their bowers are often decorated with objects purloined from humans, and the females (ergo the males) seem to have a preference for blue. Experiments show that the females prefer to mate with males whose bowers are decorated more elaborately.

Females enter and inspect the bower before mating, and the males also perform an elaborate behavioral display as well. Here’s the artist and the consumer. Note the sexual dimorphism in color, itself an indication that sexual selection is going on here:

Here is the mating display of the male satin bowerbird. Note how the female keeps a close watch on his behavior:



And, just to show how far sexual selection can go, here’s an Attenborough piece of the masterpiece of bowers—that constructed by the Vogelkop bowerbird, Amblyornis inornata.

Sexual selection is a marvelous thing, sculpting both behavior and structures like the bower and “fish circle” (things that Dawkins would call “extended phenotypes”) throughout the animal kingdom.

h/t: Matthew Cobb, Steve Pruett-Jones