A team of researchers has found that melatonin regulates “singing” behaviors in nocturnal fish. Even in constant darkness, midshipman fish vocalized at the proper times. The details are in a paper that was just published in the journal Current Biology.
Melatonin is a hormone found in most animals and many plants, bacteria, and fungi. Melatonin regulates circadian rhythms by acting as a kind of internal clock. As it begins to get darker in the evening, melatonin signals the end of the day. The hormone promotes sleep in diurnal animals, including humans. Previous research has also shown that melatonin suppresses singing behaviors in birds. The hormone also encourages activity in nocturnal animals but these effects have been understudied.
Midshipman fish (Porichthys notatus) are nocturnal, burying themselves under sand and mud during the day. At night, the males “sing” as part of a courtship routine. Researchers wondered if melatonin played a role in controlling these vocalizations.
The researchers collected wild midshipman fish to study how melatonin affected their mating songs. First, the team wanted to establish whether or not the fish’s songs were following a circadian rhythm. The fish were kept in complete darkness for a week but still sang at night. Interestingly, they began to follow a 25-hour schedule, singing an hour later each night. The team then tested the fish in a setting with constant light. In most animals, the pineal gland only produces melatonin in the absence of light. This was still true for the midshipman fish, which stopped singing until they were given a melatonin supplement. The supplement encouraged the fish to sing but the vocalizations occurred in random bursts.
The team concluded that melatonin stimulates singing behaviors in midshipman fish, the opposite of the hormone’s effects in songbirds. The findings will help researchers better understand how melatonin works in both nocturnal and diurnal animals.
Ni Y. Feng et al. “Singing” Fish Rely on Circadian Rhythm and Melatonin for the Timing of Nocturnal Courtship Vocalization. Current Biology (2016).