It has always been known by scientists that male midshipman fish make humming, or singing sounds at night, but the reason why they made the sounds was a mystery, until it was finally solved by a team of scientists from Cornell.
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Prof Andrew Bass, who led the research, said his curiosity about midshipman fish had been piqued by a paper written in 1924 by an academic called Charles Greene, which described how the male fish would hum at night.
Rumours were rife among the local human population – perhaps the droning hum came from some sinister underwater military exercise, the shifting of the tectonic plates under the sea, or maybe even some enigmatic alien that was communicating with others of its kind. They range throughout a wide area of the Pacific Ocean, “from Alaska down to Southern California”.
Male fish often do this as a courtship call, according to scientists from Yale University and Cornell University. Nocturnal animals also make melatonin when plunged into a dark night, so it clearly doesn’t put them to sleep.
Very little is known about the roles of melatonin and circadian rhythms in nocturnal vertebrates, including fish that vocalize during mating season. Scientists have unsolved the mystery behind California’s “singing fish” in a new study.
To sing, a male plainfin midshipman produces sound by vibrating a gas-filled bladder within the abdomen.
In this latest study, Dr. Feng and Prof.
While in diurnal (active during the day) creatures such as birds, melatonin brings quiet, it does the opposite in the midshipman fish. They were surprised to eventually learn that the sounds were just the mating music, or love songs, of a species of fish.
“In the case of melatonin, one hormone can exert similar or different effects in diurnal vs. nocturnal species depending on the timescale of action, from day-night rhythms to the duration of single calls”. They tried finding if the internally generated circadian rhythm was responsible for control the nocturnal singing.
To find out if the humming was controlled by an internal clock, or circadian rhythm, the team first kept a group of midshipman fish in constant light. That’s why they happen exclusively at night. Instead, they’re controlled by the fish’s internal clocks. They placed the fish in complete darkness for around a week, without any light and the fish was found singing and humming on a schedule of 25 hours, the researchers started conducting this test an hour later every night.
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. “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”, he added.
Finally, Bass and Feng located specific melatonin receptors – sites where melatonin triggers an action in the brain – in brain regions that control reproductive and social behaviors, including vocal initiation centers, the same as in birds and other vertebrates.
The study was funded by Cornell Neurobiology & Behavior Animal Behavior Grants, Sigma Xi Research Grants, and the National Science Foundation.
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Then, Bass and his colleagues gave the fish a melatonin substitute. Current Biology. September 2016. However, it turned out to be no more than the male midshipman’s nocturnal love song. Rather than telling the fish that it is time to sleep, melatonin stimulates night-time courtship humming.
