Jellyfish (or jellies) are in the phylum Cnidaria, and in the sub phylum Medusazoa. Cnidarians also include: corals, hydras, anemones, box jellies, and others. Jellies are in the class Scyphozoa. Jellies, and all other Cnidarians, are uncephalized meaning they do not have a head with a concentration of nerve cells. Cnidarians have what is called a nerve net or nerve ring that acts to carry electrical impulses. Throughout the nerve net there are concentrations of nerve cells called ganglia. Ganglia coordinates the nervous system.
The inner and outer nerve ring constitutes the nerve net.
Humans, and mammals, on the other hand are cephalized and have brains (a concentration of nerve cells). We know that mammals, and other animals, sleep but we do not know why. We have many hypotheses as to why we have to sleep. The general idea is that we sleep to allow our brains to rest, process memories, and remove toxins from cells. One study suggests that skeletal muscles also contribute to the need to sleep, at least in mice.
Claiming that all animals sleep involves changing our definitions of sleep. Also, rest must be differentiated from sleep. One can be awake and resting, but sleep is similar to being unconscious. There are two characteristics that seem universal among animals that sleep, unresponsiveness to soft noises or light movements when sleeping and a state of “sleepiness” if sleep is cut off prematurely. These criteria are met in worms, fish, and even flies which are cephalized animals.
A recent publication has provided evidence that jellies, specifically Cassiopea jellies, exhibit the criteria of sleep. The Cassiopea genus of jellies are known as the upside-down jellyfish. Their bells are directed toward the seafloor with their tentacles and mouth upward. Additionally, unlike most other jellies, Cassiopea have photosynthetic algae within their body tissues that provide nutrients. Along with that, like other jellies, Cassipoea brings in food and excretes waste via periodic pulses of the bell to bring in and release water. They typically pulsed once per second or 60 times per minute.
In the study, it was found that these jellies slowed their pulsation rate to about 39 pulses per minute during the night time. This slowing of bodily function is a good indicator of a sleep-like state. Further experiments showed a “sleepiness” in their movement when displaced from their preferred spot in the tank in comparison to when they were fully functioning or “awake”. In an additional test the researchers would deprive the jellies of “sleep” by stimulating them by pulsing water over them at varying intervals. The jellies who were more “sleep” deprived were less active the next day compared to those who were less sleep deprived and not sleep deprived. Then, as a final test, the jellies were given melatonin (a sleep aid) which seemed to put them into their sleep-like state.
If you would like to read the publication yourself, here.
Here is an article about the publication.
Here is another article which focuses more on the researchers.