ANDREW H BASS: The sounds of fishes are used in courtship and defensive behavior, just as they are in other animals, and we've been studying these behaviors in a species of fish known as the plainfin midshipman. Midshipmen are part of a larger group of sound-producing fishes known as toadfish. And each spring and summer, male midshipmen fish migrate from deep, offshore sites into the rocky intertidal where they build nests under rocky shelters. And from those nests, they broadcast an advertisement call known as a hum to try and attract females to their nest for the purposes of spawning.
Now, there are two especially prominent features of hums. One is that they're mainly only made at night. The second is that they can be especially long in duration, and a single home may last for up to two hours. And a male will make the signal repetitively throughout the night, trying to attract a female to his nest.
How do these fish control the production of this vocalization on time scales ranging from a daily rhythm down to the duration of a single call of a single hum? As we report, humming behavior exhibits an endogenous circadian rhythm that's sensitive to both light as well as to the hormone melatonin.
NI Y FENG: In the brains of vertebrates, including fish, there's a specialized organ called the pineal gland that can translate external light/dark information into an internal hormone message, and this hormone is called melatonin. Melatonin is also known as the hormone of darkness because it's produced at night in almost all species examined, regardless of whether they are active during the day or night. And its synthesis is inhibited by light.
So we wondered whether melatonin would stimulate or increase the humming activity of the nocturnal midshipman fish. To test this, we held the fish under constant light for about 10 days at a time, and we found that under constant light humming activity of midshipmen is significantly reduced. However, if we replace melatonin on the first day of constant light, their humming activity is rescued to normal levels, suggesting that, yes, melatonin serves as the go signal, or the permissive signal, for humming behavior in nocturnal midshipman fish. Surprisingly, we found the same results at a much shorter time scale of single calls, suggesting that melatonin exerts actions over widely different timescales.
ANDREW H BASS: Brain regions controlling fish vocalizations are remarkably similar in location and function to those of birds and mammals. This includes the preoptic region of the forebrain and the periaqueductal gray of the midbrain, centers for the control of reproduction and sound production in both fishes and terrestrial vertebrates. Using in-situ hybridization, we found especially robust expression of melatonin receptors in these brain regions, suggesting that melatonin's time scale-dependent effects are executed by discrete and evolutionary conserved neural pathways. Together, the evidence points to the remarkable versatility of melatonin as a hormone whose separable effects at different time scales may be selected upon over evolutionary time as dissociable modules to both pattern and coordinate social behaviors.
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Some fish sing to mate. The Andrew Bass lab has shown how melatonin can trigger this fascinating behavior.