Fish sleeping under sandy bottom: Interplay of melatonin and clock genes

Wrasse species exhibit a definite daily rhythm in locomotor activity and bury themselves in the sand at the bottom of the ocean at night. It remains unclear how their behavior in locomotor activity is endogenously regulated. The aim of the present study was to clarify the involvement of melatonin and clock genes (Per1, Per2, Bmal1, and Cry1) in daily and circadian rhythms of the threespot wrasse, Halichoeres trimaculatus, which is a common species in coral reefs. Daily and circadian rhythms in locomotor activity were monitored under conditions of light-dark cycle (LD = 12:12), constant light (LL), and darkness (DD). Daily rhythms in locomotor activity were observed under LD and persisted under LL and DD. Melatonin from a cultured pineal gland showed daily variations with an increase during the nighttime and a decrease during daytime, which persisted under DD. Melatonin treatment induced decreases in locomotor activity and respiratory rate, suggesting that melatonin has a sleep-inducing effect. Per1 and Per2 mRNA abundance in the brain under LD showed daily rhythms with an increase around lights on. Robust oscillation of Per1 and Per2 mRNA expression persisted under DD and LL, respectively. Expression of Bmal1 and Cry1 mRNA also showed daily and circadian patterns. These results suggest that clock genes are related to circadian rhythms in locomotor activity and that melatonin plays a role in inducing a sleep-like state after fish bury themselves in the sand. We conclude that the sleep-wake rhythm of the wrasse is regulated by a coordination of melatonin and clock genes.