Melatonin Regulates Nocturnal Behavior in Cirriformia okudai (Annelida: Cirratulidae)

Melatonin is one of the most ancient biologically active molecules and is nearly ubiquitous across extant life forms.In marine annelids, melatonin has been detected in the brain and is synthesized by brain photoreceptors that utilize a vertebrate-type, opsin-based phototransduction cascade coupled to a light-entrained circadian clock.We recently developed an original software to analyze the nocturnal behavior of the cultured Polychaete Perinereis aibuhitensis.Here, we investigated the effects of melatonin on the nocturnal behavior of the benthic annelid Cirriformia okudai.As C. okudai is nocturnal, individuals were exposed to melatonin (10⁻⁶ M) in artificial seawater during the night, and their behavior was monitored using a video camera.Compared with P. aibuhitensis, C. okudai exhibited an exceptionally low locomotion rate, traveling only a few centimeters per hour.To quantitatively assess locomotor behavior, we tracked each worm's position over time and calculated the proportion of the body area that moved relative to the initial position.The time points at which displacement reached 60% of the initial area were determined.As a result, the total locomotion distance did not differ significantly between control and melatonintreated individuals.In contrast, before reaching 60% displacement, melatonin treatment (10⁻⁶ M) significantly reduced the frequency of movements exceeding 1% of the initial position within a 2-min interval in C. okudai.While melatonin is known to regulate the behavior of the swimming Polychaete Platynereis dumerilii, the present study demonstrates that melatonin also modulates the behavior of benthic polychaetes.