In the streams of south-central Myanmar lives a creature that could be easily mistaken for a sentient grain of rice—a grain of rice, that is, with a lot to say and a voice like a jackhammer.
At just 12 millimeters long, the transparent fish Danionella cerebrum is among the smallest vertebrates alive, but it may be the world’s loudest animal by weight. Measured underwater and at close range, male D. cerebrum’s calls reach an astonishing 140 decibels. That’s as loud as a firecracker, says Verity Cook, a researcher at Charité–Berlin University of Medicine who studies brain activity during acoustic communication. To human ears above water, the fish’s call sounds like a short chirp or a buzzy whine, and it can last more than a minute.
But why does such a tiny fish scream so loudly? Sometimes, it seems, “it’s a love song,” says University of Lisbon marine biologist Clara Amorim, co-author of a recent study on D. cerebrum in the Journal of the Acoustical Society of America. Amorim and her colleagues compared the number of eggs laid by populations of the fish in different tanks. And it was the chattiest groups—not necessarily the largest—that seemed to lay the most eggs.
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It could be that D. cerebrum’s screechy serenade encodes information about a male’s fitness, Amorim says. She notes that belting so loudly for a minute straight requires an impressive investment of energy and that females might prefer mates with vocal endurance. In some vocalizing fish species, a male’s ballad can even speed up the maturation of a female’s eggs and promote spawning.
Cook recently discovered that male D. cerebrum make their spectacular sounds by rapidly contracting muscles that force a minuscule piece of cartilage to beat like a drum against their swim bladder, an air-filled organ some fish use to control their buoyancy.
Whatever D. cerebrum is telling females with its loud calls, it also has a lot to tell scientists about how the brain works, Cook says. The diminutive fish have emerged as a promising model animal for neuroscience: they remain transparent their entire lives, and the top of their skull never closes, resulting in something like a cranial sunroof. This means that, with the help of genetic editing and fluorescent proteins, scientists can watch individual neurons literally light up as they become active, Cook explains. She says D. cerebrum’s gabbiness provides a chance for researchers to chart the neural pathways involved in vertebrates’ production and processing of sound.
D. cerebrum has the smallest brain of any living vertebrate—scarcely bigger than a poppy seed. But Cook warns that it shouldn’t be underestimated. “Even tiny fish with little brains have complex, interesting behaviors,” she says. And, it seems, love lives.