Melatonin makes midshipman fish sing

Hormone that lulls people to sleep signals underwater fish flirting

Midshipman fish

MMM MELATONIN A humming midshipman male (second from left) and a female are surrounded by two smaller sneaky males and a brood of wispy white larvae.

Margaret Marchaterre/Cornell University

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For widemouthed, musical midshipman fish, melatonin is not a sleep hormone — it’s a serenade starter.

In breeding season, male plainfin midshipman fish (Porichthys notatus) spend their nights singing — if that’s the word for hours of sustained foghorn hums. Males dig trysting nests under rocks along much of North America’s Pacific coast, then await females drawn in by the crooning.

New lab tests show that melatonin, familiar to humans as a possible sleep aid, is a serenade “go” signal, says behavioral neurobiologist Ni Feng of Yale University.

From fish to folks, nighttime release of melatonin helps coordinate bodily timekeeping and orchestrate after-dark biology. The fish courtship chorus, however, is the first example of the hormone prompting a launch into song, according to Andrew Bass of Cornell University. And what remarkable vocalizing it is.

The plainfin midshipman male creates a steady “mmm” by quick-twitching specialized muscles around its air-filled swim bladder up to 100 times per second in chilly water. A fish can extend a single hum for about two hours, Feng and Bass report October 10 in Current Biology. That same kind of super-fast muscle shakes rattle-snake tails and trills vocal structures in songbirds and bats.

Fish underside
The dots that line the underside of the midshipman fish are photophores, organs that produce light and are reminiscent of the rows of buttons on a sailor’s uniform. Margaret Marchaterre/Cornell University
Water abuzz with nighttime fish yearning is part of the California houseboat experience, Bass says. Sausalito festivals have included kazoo choruses in the midshipman’s honor. The fish’s common name comes from its luminescent spots, which reminded early biologists of buttons down the front of nautical uniforms.

Suited up and ahum, males await females cruising amid the chorus. A male sometimes half-gulps a passing female’s head in his mouth and pulls her into his cave, Bass says. If she chooses to stay, she hovers upside down in the water, laying eggs in “a beautiful monolayer” across the ceiling. The resident male, or sometimes an interfering little sneak male, releases sperm into the water for external fertilization. Egg-papering a swath of rock can take her up to 24 hours, with the male occasionally repositioning her to cover different patches. Afterward, females swim off, leaving the dad to weeks of tending the mosaic of eggs from various females impressed by his humming marathons. 

SULTRY SERENADE Male midshipman fish (Porichthys notatus), like the one in this video, can drone on for hours luring females to mate. A night of humming starts with a cue from melatonin, a hormone that can make people sleepy, researchers at Cornell University have found. N. Feng and A. Bass/Current Biology 2016

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