Wonders of Nature: The Secret Behind the Singing Fish Mystery

A new study from Cornell University has finally shed some light in the nocturnal mating song of plainfin midshipman that baffled many people living near the Pacific coastal water from Alaska to Baja, California.

The study, published in the journal Current Biology, revealed that the combination of circadian rhythm and melatonin, the hormone that helps human fall asleep, is responsible for the nocturnal humming of the fish.

"Our results, together with those of others that also show melatonin's actions on vastly different timescales, highlight the ability of hormones in general to regulate the output of neural networks in the brain to control distinct components of behavior," explained Andrew Bass, professor of neurobiology and behavior in the College of Arts and Sciences and senior author of the study, in a press release.

For the study, the researchers caught wild male plainfin midshipman and brought them to the lab. They conducted two experiments to determine the function of circadian rhythm and melatonin to their mating song.

During the first experiment, the fish were placed to a constant darkness without any light cues for seven days to determine if the fish's daily nocturnal song was controlled by internally generated circadian rhythm. On the other hand, the researchers placed the fish on constant light for ten days during their second experiment to understand the effect of melatonin on the fish's singing behavior.

On the first experiment, the fish still sung but on a 25-hour interval, suggesting that the circadian rhythm ran with a delay of one hour that causes a drift in vocal activity with respect to the 24 hour light-dark cycle.

However, the humming of the fish was suppressed in the second experiment. The suppression occurred because the pineal gland could only produce melatonin in the dark. When the researchers gave the fish a substitute for melatonin, it continued to sing but at random times of day without rhythm.

With their findings, the researchers concluded that melatonin serves as a go signal for the nocturnal mating song of the midshipman fish and constant light at a single call timescale could decrease hum duration.