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Midshipman fish can be found all along the Pacific Coast. They are known for their distinct night-time singing to attract a mate. (Source: Tribune News Service) Photo credit: Tribune News Service

It turns out that Disney’s “The Little Mermaid” may have had a grain of truth to it (besides the obvious lesson of not giving away your voice) when it showed audiences a group of fish that could sing.

A study published on Sept. 22 detailed the research conducted on the circadian rhythm of male plainfin midshipman fish.

These fish are known for singing in order to attract mates and will typically begin singing late at night and continue through till the early morning. Midshipman fish are found all along the pacific coast and were first discovered by people on houseboats in the San Francisco Bay.

They heard a hum that started late each night and ended in the morning.

Researchers found that melatonin affected the nocturnal fish in an unusual way, acting as a signal for the fish to begin singing at night.

The results were compared with those of a previous study done on daytime songbirds, which found that melatonin suppressed the songbirds’ urge to sing at night.

Andrew Bass, a professor of neurobiology and behavior, was the senior author of the paper.

“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,” Bass said.

The fish were studied in a lab so that the researchers could control the lighting as needed. For one stage of the research, the fish were placed in complete darkness for a week.

Despite the lack of light, the fish still sang on a schedule, though it was a 25-hour schedule instead of 24, meaning the fish began an hour later than they typically did each night.

We found that this clock ran with a delay of about an hour, causing a drift in vocal activity with respect to the 24-hour light-dark cycle, highlighting the importance of internal clocks to be recalibrated by environmental cues on a daily basis,” said Ni Feng, postdoctoral researcher at Yale and first author on the paper.

In contrast to the experiment conducted in complete darkness, the fish were also placed in a constantly-lit tank for 10 days at a time. The fishes’ humming was not as intense or frequent since they weren’t producing melatonin.

Researchers gave the fish a melatonin substitute, which increased humming but had no effect on the rhythm, leaving the fish humming at sporadic intervals during the day.

“Melatonin acts as a ‘go’ signal for the nocturnal call of the midshipman fish,” Feng said. “Surprisingly, at the single call timescale, constant light also decreased hum duration, but melatonin maintained hum duration at normal levels, a finding also found in diurnal birds.”

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