Mechanisms regulating circadian and sleep behavior in the upside-down jellyfish Cassiopea Andromeda

All animals had to adapt to our planet`s rotation and the day-night cycle, and evolve endogenous pacemakers that drive rhythmic physiology and behavior. The circadian clock is entrained by environmental inputs, such as light and temperature, and regulates rhythmic outputs including the timing of sleep. Sleep is an evolutionarily conserved behavior in all animals, which is regulated by homeostatic and circadian processes. Sleep is crucial to the health of the nervous system and regulates various processes ranging from cellular maintenance to memory and learning. However, why this behavior, which puts the animal under the risk of predation, has evolved, is unclear. Recently, behavioral criteria were used to define the sleep-like state in cnidarians, which are lower basal animals with a simple nerve net. In this study, we utilize the upside-down jellyfish Cassiopea to understand the origin and mechanisms of the circadian clock and the sleep/wake cycle. Cassiopea is a diurnal jellyfish that harbors the photosymbiont Symbiodinium microadraticum (Dinoflagellata; Symbiodiniaceae). We hypothesize that interactions between light, algae photosynthesis, photosymbiosis, ROS, and DNA damage, regulate rhythmic behavior and sleep pressure in Cassiopea. Using custom-made behavioral systems, computational analysis, immunohistochemistry, and molecular analysis, we characterize circadian and sleep mechanisms in both the single neuron and whole organism levels. The results are expected to reveal how sleep has evolved and suggest why it is conserved across phylogeny.