Do jellyfish dream of gelatinous sheep? We can’t say, but according to a paper available in the journal Current Biology, they do seem to actually sleep, or at least enter a sleep-like state.
This discovery began with three graduate students at Caltech. Michael Abrams and Ravi Nath were debating whether or not jellyfish went to sleep at night. Their colleague, Claire Bedbrook, overheard and commented that it is a complex if passive, behavior. So jellyfish, known for their lack of brains, obviously could not. They decided to test the question, and Bedbrook has since changed her stance.
Jellyfish Go to Rest at Night and Don’t Like Disruptions
The team of researchers discovered that such specimens were 30 percent less active at night, and could be “woken up” with some difficulty. If kept from sleeping, they spent the next day dazed, meaning that sleep is biologically necessary for the species.
This odd behavior was seen in a species called Cassiopea, or the upside-down jellyfish. Instead of brains, they have a loose network of nerve cells throughout their bodies. Bedbrook notes that they are “like weird plant animals,”. Ones that seem to have a symbiotic relationship with photosynthetic organisms in their cells.
Jellyfish like Cassiopea are also some of the oldest known creatures, dating back 700 million years. For perspective, the oldest dinosaur would show up more than 450 million years later. This odd discovery helped awaken and give rise to a series of questions. Some of them target the nature of sleep and its possibilities when lacking a central nervous system. Other question which animals that lack a brain can also enter a sleep-like state.
Prior to this, experiments had already shown that creatures as simple as roundworms, C. elegans, need sleep to survive. This new find shows that this requirement apparently showed up even earlier in our evolutionary history that initially believed.
But as Nath notes, it is a rather counter-intuitive idea, “this period where animals are not doing the things that benefit from a natural selection perspective.”
“We know it must be very important. Otherwise, we would just lose it,” Bedbrook noted.