Jellyfish are interesting for lots of reasons, one of them being their place on the evolutionary landscape. The first jellyfish-like creatures (the ancestors of modern jellyfish) pitched up during the Cambrian explosion, more than 500 million years ago and are one of the first animal groups to boast organised tissues and a nervous system. There is nothing that could be described as a brain, but jellyfish neurons work in exactly the same way as vertebrate neurons. So do they experience anything akin to vertebrate sleep?
The definition of sleep, at least as espoused by the Oxford English Dictionary, is “to take repose by the natural suspension of consciousness”. If jellyfish don’t have a brain, they probably are not capable of consciousness and should therefore not be capable of sleep. But there’s intriguing evidence that jellyfish go through periods of relative activity and inactivity. If this is the case, it could shed light on the original function of quiescence and hence the evolution of sleep.
The box jellyfish Chironex fleckeri is a species infamous for its lethal sting. They are active predators and are extremely mobile during the hours of daylight, typically covering around 200 metres an hour. At night, however, they basically stop moving altogether. ‘During these periods of “inactivity”, the jellyfish lie motionless on the sea floor, with no bell pulsation occurring and with tentacles completely relaxed and in contact with the sea floor,’ wrote Jamie Seymour, a biologist at James Cook University in Cairns, in the Medical Journal of Australia back in 2004. A small disturbance – like a light or a vibration – ‘causes the animals to rise from the sea floor, swim around for a short period, and then fall back into an inactive state on the sand.” To Seymour and his colleagues, this looked a lot like sleep.
What is particularly interesting about the box jellyfish is that it has four sets of six eyes (so 24 in total), some of them endowed with lenses and retinas and are clearly capable of forming images. Perhaps these structures, which require huge neurological processing, set the stage for the emergence of two distinct states of vigilance. One – wakefulness – allowed the animal to focus on the analysis of complex visual information and the split-second making of decisions. The other – sleep – became the brain’s opportunity to process information without being overloaded by the senses. It’s an idea put forward by evolutionary biologist Lee Kavanau in the 1990s.
In 2017, researchers addressed the question of whether jellyfish sleep in a more robust manner. Working with the upside-down jellyfish of the genus Cassiopea in a laboratory setting, they used cameras to extract information on pulsing activity and found them less active during the hours of darkness. When released into a water column during hours of light, the jellyfish were pretty quick to pulse (within 2s). In darkness, however, it took them much longer to respond to the disturbance (around 6s). If the intervention had indeed “woken” the animals from a kip, the researchers predicted they would be quicker to respond to another whoosh through the water just 30s later. This turned out to be the case, from which we can infer that a jellyfish – unceremoniously disturbed from its night-time slumber – takes more than 30s to fall back asleep. A further experiment showed that when deprived of rest (by regularly mixing the water column), the jellyfish catch up on it as soon as they can, strongly suggesting it’s possible to sleep-deprive a jellyfish. Unkind perhaps, but interesting.
There are so many similarities between jellyfish sleep and that of vertebrates – at a molecular, physiological and behavioural level – it looks like this sleep may only have evolved once, being modified through natural selection to meet the behavioural and ecological needs of those species that came later.
In most vertebrates, including humans, sleep is now thought to bring many benefits, allowing the strengthening of some synapses, the paring back of others, the replenishment of neurotransmitters, the removal of toxins, repair of ware and tare and energy saving. I think we can conclude that jellyfish do sleep or something very like it. What will be really interesting is to study the variation in this behaviour in different lineages of jellyfish, as this might shed light on what benefits this quiescence brought to the most primitive jellyfish way back in the mists of evolutionary time.