In 1924, an academic called Charles Greene described how the “California singing fish” would hum at night. Just why the plainfin midshipman is so vocal at night remained a mystery for nearly a century, until now.
For much of the year, you won’t hear these fish singing at all. The plainfin midshipman, named after the bioluminescent organs on its underside, which reminded early observers of uniform buttons, resides in the depths of the ocean during the fall and winter. During the spring and early summer, they move to coastal waters between Alaska and Baja California. There, the male fish “sing” to attract mates, a sound that can be heard by humans onshore.
But these vocalizations aren’t spontaneous, say Cornell University researchers Andrew Bass and Ni Feng in a new study in Current Biology. Instead, they’re controlled by the fish’s internal clocks. That’s why they happen exclusively at night. And the hormone that controls these clocks is the same one that regulates bird activity and human sleep patterns.
“Circadian rhythms govern the daily lives of diverse lineages, from plants to animals,” Dr. Feng, the first author of the paper, a former graduate student in Professor Bass’ lab who is now doing postdoctoral research at Yale, told the Cornell Chronicle. “Our study helps cement melatonin as a timing signal for social communication behaviors.”
To sing, a male plainfin midshipman produces sound by vibrating a gas-filled bladder within the abdomen. Each hum, intended to draw females to the nest he has prepared, can last up to two hours – and males sometimes hum together, magnifying the sound.
The scientists caught a number of wild male plainfin midshipman fish and kept them in labs where the brightness could be controlled. They found that, when the light was constantly bright, the fish did not secrete melatonin and did not hum. With bright light, fish given a melatonin substitute would hum, but at random times, suggesting that the humming is a reaction to the melatonin.
In nature, when it goes dark, a plainfin midshipman fish’s melatonin levels go up, waking him up and causing him to sing.
Melatonin has the opposite effect on birds and mammals that are active during the day: it helps them to stay quiet and fall asleep. But the study establishes that the same hormone is the driving force behind circadian rhythms across nature.
Almost all animals secrete melatonin, according to Feng. "Our study shows that singing fish can be a useful model for studying hormones and reproductive-related vocal communication behaviours shared by many vertebrate species," she explained to the BBC.
For more information on the plainfin midshipman, check out this video from the Elkhorn Slough Foundation.