The Cassiopea jellyfish has no brain, no spine, and spends most of its life lying upside down on the seabed. Yet scientists have now discovered that this simple animal appears to sleep—suggesting that sleep may be far more ancient than previously thought.
Cassiopea jellyfish live in shallow coastal waters such as mangrove forests, where they rest upside down on the ocean floor. Researchers at the California Institute of Technology (Caltech) observed that these jellyfish become significantly less active at night, entering a state that closely resembles sleep.
“This discovery shows that even animals without a central nervous system need sleep,” said Ravi Nath, one of the study’s authors. “This suggests that sleep may be one of the most ancient behavioral states in the animal kingdom.”
The findings were published in the scientific journal Current Biology.
Cassiopea jellyfish are believed to be among the earliest animals to evolve nerve cells and possess one of the simplest nervous systems known. While sleep has been documented in animals with more complex nervous systems, researchers had never previously tested whether jellyfish experience sleep-like behavior.
To investigate, the scientists placed jellyfish in aquariums and monitored their movements using cameras. They found that the jellyfish pulsed far less frequently at night. During the day, Cassiopea typically contracts its bell around 58 times per minute, but at night the rate drops to about 39 contractions per minute.
Although the jellyfish continue to move at night, the slower rhythm suggests a sleep-like resting state.
The researchers conducted additional experiments to confirm this behavior. In one test, they placed jellyfish on a platform in the aquarium and waited until they appeared inactive. When the platform was suddenly removed, alert jellyfish during the daytime quickly swam downward to the bottom. However, jellyfish that appeared to be sleeping took several seconds longer to respond, drifting before reorienting themselves.
In another experiment, the scientists gently disturbed the jellyfish at night to prevent them from resting. When deprived of sleep during the night, the jellyfish later showed signs of compensating by entering sleep-like states during the day.
“We do not yet know why animals need sleep,” Nath explained, “but our study provides clues about the ancient origins of sleep.”
The findings suggest that sleep may have evolved very early in the history of life. If even simple animals such as jellyfish require sleep, it is possible that some of the reasons humans sleep today are rooted in these ancient biological processes.
Reference:
Nath, R. et al. “The Jellyfish Cassiopea Exhibits a Sleep-like State.” Current Biology. DOI: 10.1016/j.cub.2017.08.014.