Despite the fact that jellyfish lack a centralized nervous system, new research provides strong evidence that these soft-bodied marine organisms sleep. The findings, published in the journal Current Biology, have scientists reevaluating what sleep is and what it does.
“Some people probably think that a brain is needed for a sleep behavior to exist, especially those scientists that focus on the sleep states of mammals,” Michael Abrams of Caltech, who co-led the study with his Caltech colleague Ravi Nath, told Seeker.
“Though there are anecdotal observations that suggest some species don’t sleep, I do not know of any animal that has been fully interrogated that has been proven not to sleep,” he added.
Nath and fellow researchers from three Caltech laboratories — headed up respectively by Paul Sternberg, Viviana Gradinaru, and Lea Goentoro — decided in this case to investigate Cassiopea, aka the Upside-Down Jellyfish. This primitive genus essentially spends its entire life sitting upside down on the ocean floor, pulsating every few seconds.
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Three criteria must be met in order for an organism to be considered as “sleeping.” First, it must demonstrate a period of reduced activity. To test this in the jellyfish, the researchers used cameras to continuously monitor the marine dwellers in tanks. The footage revealed that jellyfish go through periods of inactivity at night, only pulsing about 39 times per minute, compared to about 58 times per minute during the day.
“During the jellyfish sleep-like state, we see a reduced number of pulses, and we also see an increase in the frequency and length of pause events,” co-lead author Claire Bedbrook told Seeker.
“Pause events are where the jellyfish stop pulsing altogether for 4–20 seconds. These pause events are rarely seen during the day, when the jellyfish are in their more active state.”