If you think that fish can't sing, then you've probably never come into contact with a midshipman fish. When it's time to breed, male midshipman hum repetitively at night as they attempt to woo females into their nests for spawning. Now, researchers reporting in the Cell Press journal Current Biology on September 22 have new evidence to explain what keeps those fish quiet through the day and singing all night.
The keys are the fishes' light-driven internal clock and melatonin, a hormone best known for controlling sleep and wake cycles.
"For some observers, the midshipman's courtship 'song'--the hum--is reminiscent of the sound of a foghorn, but more enticingly it reminds us of the droning sounds made by Mongolian throat singers," says Andrew Bass of Cornell University.
A single hum can last for minutes up to almost two hours. When neighboring males hum together in a chorus, slight differences in hum frequencies make the sound even more otherworldly.
"In the early 1980s, a mysterious sound caused concern for houseboat residents of Sausalito Bay, California, who suspected the source might be the pumps of a nearby sewage plant, an underwater power line, some secret experiment by the Navy, or maybe even extraterrestrials!" Bass says. "It turned out their houseboats were merely resonating with the 'love songs' of male midshipman fish."
Bass and Ni Y. Feng, a former graduate student in the Bass lab, knew that the midshipman fish kept the rocky intertidal zone humming during the breeding season. To find out why the fish only sing at night, they first needed to get them to hum in the lab, where they could control the lighting and other factors.
The researchers' studies now show that the fish continue to hum on schedule when kept in conditions of constant darkness. But their humming is suppressed when the lights are kept on, a condition known to reduce the production of melatonin. Indeed, when a group of fish kept in the light were given melatonin, their humming stayed at normal levels and even increased.
"While our brain studies predicted melatonin would increase humming, it was still striking to see the robust melatonin effect that rescued the suppression of singing induced by exposing the fish to constant light," Feng says.
The researchers went on to show evidence that melatonin acts directly on the vocal circuitry in the midshipman's brain.
The findings show the remarkable versatility of melatonin as a timing signal, the researchers say. Melatonin keeps birds quiet at night, but in the midshipman fish melatonin has just the opposite effect.
Despite that difference in melatonin's effect on vocal activity between diurnal birds and nocturnal midshipman fish, there are similarities, too. In both species, constant light and reduced melatonin action shortens the duration of individual vocal elements.
The researchers say that they'd like to continue to explore the molecular-level events in the brain that lead melatonin to act in opposing ways in diurnal versus nocturnal species. They also hope that their findings will inspire others to find out whether melatonin plays a role in the late-night calls of other animals, such as nightingales and many species of frogs and mammals.
This research was funded by the National Science Foundation, a Cornell Neurobiology and Behavior Animal Behavior Grant, and a Sigma Xi Research Grant.
Current Biology, Feng and Bass: "'Singing' Fish Rely on Circadian Rhythm and Melatonin for the Timing of Nocturnal Courtship Vocalization" http://www.cell.com/current-biology/fulltext/S0960-9822(16)30873-9
Current Biology (@CurrentBiology), published by Cell Press, is a bimonthly journal that features papers across all areas of biology. Current Biology strives to foster communication across fields of biology, both by publishing important findings of general interest and through highly accessible front matter for non-specialists. Visit: http://www.