Can Cassiopea Jellyfish Sleep?


An experiment was conducted to test whether Cassiopea Jellyfish, or the upside-down jellyfish, can actually experience sleep. This experiment was conducted by three graduate students of Caltech: Michael Abrams, Claire Bedrook, and Ravi Nath in a lab at the California Institute of Technology, during midnight. The students first indication that these jellyfish could sleep was when the lights were turned off and the jellyfishes’ pulse immediately began to slow. The Cassiopea jellyfishes’ pulsing was infrequent and they were staying still for long periods of time.

The Cassiopea Jellyfish don’t have brains. They have a diffuse nerve net, which is a system of neurons that are spread throughout an organism and not organized around a brain. The Cassiopea jellyfish don’t even act like typical animals. For example, instead of mouths, they suck in food through pores in their tentacles. They get energy from symbiotic relationships with tiny photosynthetic organisms that live inside their cells. “They’re like weird plant animals”, Bedrook said. Although, when the jellyfish are exposed to melatonin they get sleepy, just as humans do. This is a hint that their underlying sleep mechanism may be similar to ours.

Their experiment was to see if Cassiopea Jellyfish could sleep.  The three students conducted the research which led to the discovery that sleep is a complex behavior associated with memory consolidation and REM cycles in the brain. Their conclusion to the experiment was that Cassiopea Jellyfish do in fact experience sleep like behavior. This finding was the first documented example of sleep like behavior in an animal that has a diffuse nerve net instead of a brain. It also makes them the first brainless animals to experience sleep. These findings were published to the journal of Current Biology, and further study of sleeping jellyfish might bring scientists closer to resolving what Nath calls “the paradox of sleep”.

When conducting the experiment the three students tested for three pieces of behavioral criteria associated with sleep. The first criteria was reversible quiescence. Or in simpler terms, the jellyfish become inactive but are not paralyzed nor in a coma. The three researchers monitored 23 jellyfish, over 6 days, and counted the jellyfish’s movements. They found that the jellyfish were 30% less active at night. Although, when the researchers dropped food in the tank the creatures perked right up, clearly not paralyzed. The second criteria was an increase in arousal threshold. This means that it’s more difficult to get the animals’ attention, or they have to be woken up. For this part of the experiment the researchers placed the sleeping jellies in containers with removable bottoms. They lifted the containers to the top of the water in the tank and pulled out the bottom of the container. If the jellyfish were awake they’d automatically swim to the bottom of the tank; but if they were asleep “they’d kind of strangely float around the water”, Abrams said. When the researchers did this during the day the creatures would immediately swim to the bottom. At night, however, they would sluggishly float around. The upside-down jellyfish get their name from the fact that they sit upside down on the seafloor, they don’t like to be suspended in water. The last criteria is that the quiescent state must be homeostatically regulated. Also know as, the jellyfish must feel a biological drive to sleep. When they don’t they suffer. To test this, the researchers pulsed water through the tank every 20 minutes at night to prevent the jellyfish from sleeping. The following day and night, the jellyfish exhibited lower levels of activity than normal; sleep deprivation.

The results of this experiment suggests that sleep is deeply rooted in our biology, a behavior that evolved early in the history of animal life and has stuck with us ever since. Cnidarians, the phylogenetic group that includes jellies, first arouse some 700 million years ago, making them some of Earth’s first animals. “Sleep is this period where animals are not doing the things that benefit from a natural selection perspective”, Nath said. If animals could evolve in a way to live without sleep they surely would have. “We know it must be very important. Otherwise, we would lose it”, Bedrook said.