Mystery solved: Melatonin makes these fish sing at night

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There are fish that sing. Or drone like a bunch of loud kazoos, anyway. People living in houseboats in the San Francisco Bay have grown used to a low, strange hum that begins suddenly in the dark of the night and stops just as abruptly in the early morning.

The noise comes from male suitors of the species Porichthys notatus, commonly called the plainfin midshipman fish. While female midshipman only grunt when showing aggression, the males trying to mate with them use their swim bladders to create noises that have been likened to a chorus of kazoos, a formation of flying jets or a swarm of droning bees.

“They sound like an orchestra full of mournful, rasping oboes,” SFGate reported in 2004.
(giphy)Researchers now believe they’ve figured out how the midshipman keeps his crooning so punctual: melatonin. Melatonin is a hormone produced by many plants and animals, its release triggered by darkness. In animals that sleep at night — as humans do — melatonin is thought to help regulate the internal “clocks” that tell our bodies it’s time for some shut-eye. Some humans even find respite from insomnia by taking melatonin supplements at bedtime.

But the role of melatonin in the lives of nocturnal vertebrates remains quite mysterious. Nocturnal animals also make melatonin when plunged into a dark night, so it clearly doesn’t put them to sleep.

The new study published in Current Biology suggests that melatonin may act as more of a signal to trigger nighttime behaviors than a sleepy-time chemical. In midshipman fish, it seems to prompt males to start singing — which is what makes them midnight crooners.

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