What Makes These Fish Sing All Night Long?

Study reveals melatonin triggers the midshipman fish’s nocturnal mating song.

Between dusk and dawn, all along the Pacific Coast, males of one species of fish called the midshipman sing to their heart’s content. Their low, droning night-time courtship call puzzled houseboat residents in the San Francisco Bay in the 1980s.

New research published in the journal Current Biology shows that melatonin — the hormone that helps regulate sleep and wake cycles — is also responsible for the night chorus of this nocturnal fish.

Previous studies have revealed that in songbirds, which sing in the daytime, melatonin suppresses singing at night but triggers longer calls during the day.

To understand melatonin's effects on the singing midshipman fish, researchers brought a few subjects into the lab, where they were kept in constant light for 10 days at a time. Animals like fish and humans only produce melatonin in the dark, so the finding that the lack of darkness suppressed the fish’s song suggested that the hormone was an important driver of this singing behavior.

Further, when the dark-deprived fish were given a melatonin substitute, they went back to humming — though they had lost their usual rhythm, and would perform at random times throughout the day.

"Melatonin acts as a 'go' signal for the nocturnal call of the midshipman fish," study lead author Ni Feng, a postdoctoral researcher at Yale, said in a press release. "Surprisingly, at the single call timescale, constant light also decreased hum duration, but melatonin maintained hum duration at normal levels, a finding also found in diurnal birds."

The researchers also discovered that the fish’s melatonin receptors are located in the brain regions responsible for reproductive and social behaviors, including vocalization, thus providing a direct link between the hormone and the fish’s courtship song.

"Melatonin is an ancient and multifunctional molecule that is found almost ubiquitously in the animal kingdom," Feng explains.

"Our study helps cement melatonin as a timing signal for social communication behaviors."

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