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takyon [soylentnews.org] writes:

The singing of midshipman fish [wikipedia.org] is linked to the melatonin cycle [bbc.com]:

The researchers found the singing was controlled by a hormone that helps humans to sleep - melatonin. And looking more closely at how melatonin acts on receptors in different parts of the fish's brain could help explain why it is such a powerful "chemical clock" with a role in the timing of sleep-wake cycles, reproduction and birdsong. Prof Andrew Bass, who led the research, said his curiosity about midshipman fish had been piqued by a paper written in 1924 by an academic called Charles Greene, which described how the male fish would hum at night.

[...] To find out if the humming was controlled by an internal clock, or circadian rhythm, the team first kept a group of midshipman fish in constant light. This almost completely suppressed their humming. "But when [we gave the fish] a melatonin substitute," said Prof Bass, "they continued to hum, though at random times of day without a rhythm. "Melatonin essentially acted as a 'go' signal for the midshipman's nocturnal calling."

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