New York, Sep 26 (IANS): Certain fish keep singing through the night to attract mates and researchers have found how melatonin, a time-keeping hormone, and daily light cycles help the fish keep a tab on the timing of their humming -- starting in the late evening and stopping suddenly in the morning.

Male plainfin midshipman fish (Porichthys notatus) sing at night to attract mates, but very little is known about the roles of melatonin and circadian rhythms in nocturnal vertebrates, including fish that vocalise during mating season.

Other studies on diurnal (day-active) songbirds have shown that melatonin suppresses singing at night but increases the duration of syllables when these birds do sing.

In the current study, in the journal Current Biology, the researchers found that melatonin had an opposite effect on these nocturnal fish compared with diurnal birds.

Its release provided a "go signal" for night singing. But similar to diurnal songbirds, the hormone also acted to lengthen calls when the fish sang.

"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 behaviour," said senior author Andrew Bass, Professor Cornell University in New York.

"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," Bass noted.

In the study, the researchers brought wild-caught midshipman fish into the lab, where they could control lighting.

In one experiment, they tested if the male fish's daily nocturnal song was controlled by an internally generated circadian rhythm.

They put the fish in constant darkness without any light cues for seven days at a time, and found the fish still sang but on a 25-hour schedule, so they started one hour later each night.

In another experiment, to understand melatonin's effects on behavior, fish were exposed to constant light for 10-day stretches.

The pineal gland produces melatonin in vertebrates but only in the dark, and constant light significantly suppressed the fish's humming. But when fish were given a melatonin substitute, they continued to hum, though at random times of day without a rhythm, the study said.

The researchers also located specific melatonin receptors -- sites where melatonin triggers an action in the brain -- in brain regions that control reproductive and social behaviours, including vocal initiation centers, the same as in birds and other vertebrates.