The Pacific coast of North America is home to a truly peculiar toadfish, the plainfin midshipman (Porichthys notatus). Male midshipman fish nest under rocks in the intertidal zone and ‘sing’ to attract female mates to their nests. Midshipman fish only sing their courtship song at night, and the song is a low, continuous hum (∼100 Hz) that males make by twitching specialized sonic muscles on their swim bladders. These muscles can twitch at an amazing rate, upwards of 100 contractions per second, which is one of the fastest twitching muscles in any vertebrate.
But how do midshipman fish know to sing their songs at night? Like songbirds that call during the day, do midshipman fish use hormonal cues to govern their vocalizations? These questions captured the attention of Ni Feng and Andrew Bass, from Cornell University, USA. The pair set out to examine whether melatonin, a time-keeping hormone in vertebrates, might control midshipman song.
Melatonin production follows a circadian rhythm that translates the light–dark signal of the rising and setting sun into an internal hormonal signal. The role of melatonin is often inhibitory in day-active, diurnal animals such as songbirds: when melatonin concentrations increase after sunset, animal activity and song production are suppressed. Melatonin provides animals with cues about when to be active and when to rest during their daily cycle.
Feng and Bass tested whether the nocturnal courtship call of the midshipman was under circadian control and stimulated by melatonin. The pair of investigators experimentally changed external light cycles in the male midshipman’s tanks using programmable lighting and altered their internal melatonin levels using implants that would mimic melatonin's action. Feng and Bass then recorded how male courtship song changed under these varying conditions.
Under normal light–dark conditions, the researchers found that midshipman song had a circadian rhythm and the fish would only sing at night. However, when the researchers placed the fish under constant light, their daily rhythm of singing was suppressed. Constant light inhibited their singing cycles, likely because melatonin was also suppressed. Next, Feng and Bass implanted midshipman fish with a melatonin mimic to test whether they could ‘rescue’ male song under constant light, by adding a melatonin cue, and this time the fish began singing again. The hormone can trigger singing, even in daylight.
But how does melatonin cue the midshipman's reproductive ballads? To answer this final question, Feng and Bass mapped melatonin receptors in the brain of midshipman fish. They found that melatonin receptors were indeed concentrated in areas of the brain that govern reproductive behaviours and song production. This map nicely linked melatonin to the daily regulation of such behaviours.
Feng and Bass have interestingly shown that melatonin acts in the opposite direction to most diurnal animals: it actually stimulates song production at night in these nocturnal fish, rather than suppressing it. Their elegant series of experiments showed that regular cues from melatonin ensure that midshipman fish keep their nightly courtship serenades on time.