Cnidarian Sleep

Perhaps nothing is stronger evidence of the importance of sleep than its conservation across animals. Even cnidarians, gelatinous aquatic animals such as the jellyfish Cassiopea and the polyp Hydra exhibit the hallmark features of sleep observed in animals with centralized nervous systems: reversible quiescence, increased latency to arousal, and homeostatic regulation. Despite lacking a brain, cnidarians possess muscles controlled by neural networks and neurotransmitter systems, including GABA, dopamine, acetylcholine, and melatonin. Further, conserved sleep regulators, including the cGMP-dependent protein kinase 1, PRKG1, and GABA, in Hydra, and a nicotinic cholinergic receptor alpha subunit, CHRNAL-E, in Cassiopea, emphasize that not only is the behavioral state conserved, but so too are components of the regulatory system. We also discuss possible functions of sleep in Cnidaria, including roles in cell proliferation and the homeostasis of neurological pacemaker activity. Overall, investigations in cnidarian sleep have shed light on fundamental aspects of sleep regulation and underscore the importance of studying these organisms to gain insight into the evolutionary origins and basic mechanisms of sleep.