The "moonlight sonata" of a certain fish species in the Pacific is considered as one of the marvels of nature, mostly because it can only be heard at night.

Now, researchers in the United States claim to have unlocked the secrets behind the singing of the fish species. Apparently, the phenomenon is linked to a hormone that helps human maintain sleep.

The Midshipman Fish

In the 1980s, California houseboat occupants would often hear a strange, submarine-like hum late at night. This has led them to wonder whether the noise might be coming from military experiments, sewage pumps, or even extraterrestrial origins.

But scientists discovered that the low hum is actually something else: it's the "nocturnal love song" of a male species of plainfin midshipman fish as it woos females during breeding season.

The plainfin midshipman can grow up to 38 centimeters or 15 inches long, and typically has olive-brown color. The fish's underside contained rows of bioluminescent organs that reminded observers of the buttons on the uniform of a midshipman — hence, the name.

In a new study, experts brought the plainfin midshipman to their laboratory and conducted experiments to determine what makes these fish sing only at night.

Rhythm And Light

Led by Professor Andrew Bass, the team first kept a small group of midshipman fish in constant darkness, where it evidently hummed in rhythm.

On the other hand, when the fish was kept in constant light, their humming was almost completely suppressed, researchers said.

Then, Bass and his colleagues gave the fish a melatonin substitute. When this was done, the fish continued to hum, although at random times of day and without a rhythm, said Bass.

The team's analysis revealed that the vocalization of the fish, which resembles a low-frequency hum like that of a foghorn, is controlled by melatonin, as well as a light-driven internal clock.

This suggests that melatonin, which governs sleep and wake cycles in humans, essentially acts as a "go" signal for the nocturnal singing of the midshipman fish.

Dr. Ni Feng of Yale University, one of the researchers of the study, says melatonin helps humans fall asleep faster and alleviates jet lag. Among midshipman fish, the effect is different.

"[I]t serves to wake them up and pave the way for their nocturnal courtship song performance," said Feng.

Benefits Of The Courtship Song

Bass and his colleagues believe that the singing and hearing of the midshipman fish plays a major role in their reproductive habits and social interactions.

For instance, the humming might be scheduled for when female species are most receptive, or for when the predators are less likely to hear sounds, researchers said.

Furthermore, the research reveals a fundamental role for the hormone melatonin in the animal kingdom. A fish with behavior that is tied to melatonin and their circadian rhythm may indicate that the brain circuitry evolved in our aquatic and primitive ancestors.

“Our study helps cement melatonin as a timing signal for social communication behaviors," added Feng.

Details of the study are published in the journal Current Biology.