But this was the nocturnal hum of the midshipman fish; a courtship call, and the source of a biological secret scientists have now solved. Researchers brought the fish into their lab to work out why they sang at night. The US team's findings are published in the journal Current Biology . Chemical clock 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. "Greene referred to midshipman as the California singing fish," said Prof Bass. "We discovered that females are also sonic, but it's only territorial males that build nests and produce the hum to attract females to [those] nests." Nocturnal calling Image copyright A Bass Image caption Midshipman got their name because the luminescent 'photophores' reminded early observers of the buttons on a naval academy midshipmans uniform 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.
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