Researchers at the California Institute of Technology have found evidence that at least one type of jellyfish engages in a very unexpected behavior: sleep. The study, published by Ph.D. candidate Ravi Nath and his fellow researchers in Current Biology  in September, showed that Cassiopea jellyfish passed several criteria established in their lab to demonstrate they were engaging in a behavior that could be considered sleep.
The finding comes as a surprise to the scientific community, as previously sleep was thought to be an activity performed only by more complex organisms with central nervous systems: humans, dogs, fish, even worms. Now, adding to the mystery of why organisms sleep, there is one without a brain that does it too.
To establish that the jellyfish were sleeping rather than engaging in other behavior, the researchers set up three criteria: a regular period of diminished activity, decreased responsiveness to stimuli during this period and an increased need for the hypothesized sleep behavior when it was not getting enough. The jellyfish passed all three.
Formal testing revealed that the jellyfish pulsed 30% less during this period of diminished activity and could be “awoken” with food or prodding, ruling out other possible states such as coma. The researchers tested responsiveness by removing the floors from under the jellyfish at random times; in the hypothesized sleeping phase, they would float around before swimming to their preferred place on the floor of the tank. A need for sleep was operationalized by shooting water through the tank every 20 minutes, keeping the jellyfish from attaining this restful state; during the wakeful period the following day, the jellyfish engaged in lower levels of activity than usual.
Cassiopea, the “upside-down jellyfish” have a non-centralized radially symmetric nerve net, a diffused organization of nerve cells throughout the body with no large centralized concentration (a brain) . However, like organisms with central nervous systems, theirs functions using action potentials, synaptic transmission, neuropeptides and neurotransmitters. This commonality suggests maintenance of the nervous system at a very basic level may be a reason organisms need to sleep.
Cnidaria, the phylum of Cassiopea, branched early on from the evolutionary line of human beings. The researchers suggest that this signifies “sleep is rooted in basic requirements that are conserved across the animal kingdom.”  More research however, will need to be done to determine whether this behavior evolved in Cnidaria separately, or whether it is truly an early behavior in our evolutionary history.
Image source: prilfish via Flickr
 Nath et al., Curr Biol, 2017;
Originally published in Charité Neuroscience Newsletter, December 2017: Vol. 10, Issue 4