Sleeping is wide-spread in the animal kingdom, from hibernating bears to micro-napping bullfrogs. Even insects, worms and jellyfish sleep! The changes in brain state associated with sleep are critical for healing, cellular repair, processing and forming memories. Yet, studies assessing the role of sleep and its mechanisms in fish are quite sparse. You might be surprised to learn that shark do indeed sleep! So is their sleep similar to our's? How long do they sleep for? And how do they keep breathing whilst they slumber?
Sleeping on It
Scientists are only just beginning to understand how and when sharks sleep and there is a lot we still don't know yet. It has been a challenging field of study because sharks are so elusive and they live in an environment that is difficult for human beings to work in. Whatsmore, it is also very difficult to record brain wave activity in water. What we do know about sleep in sharks has been deduced from the study of only a small number of individuals or from anecdotal evidence, but it does appear that sharks do sleep! (Kelly et al, 2021).
Just Keep Swimming
Many people assume sharks cannot sleep, thanks to the pervasive myth that if a shark stops swimming it will die. This idea comes from overgeneralisation and poor knowledge of shark respiratory physiology. Whilst there are some species of sharks that do need to keep moving to "ventilate" (pass oxygenated water over their gills), there are far, far more sharks that do not and can rest quite happily on the seafloor for long periods of time.
Sharks that must keep swimming to breathe, like hammerheads (Family Sphyrnidae), are known as "obligate ram ventilators". Species that can stay still and keep breathing do so via a process known as "buccal pumping", where they move the muscles of their mouth to draw water in and over their gills whilst remaining stationary. Some species can even switch between the two methods; using buccal pumping when at rest and ventilating via "ram ventilation" whilst swimming around. To learn more, you can check out Myth Busted: Sharks DO NOT Have to Keep Swimming to Breathe.
There is evidence that both buccal-pumping and ram ventilating sharks species do sleep, or at least rest! However, this sleep is not necessarily like that which humans experience and can vary enormously between species (Kelly et al, 2019).
Mouth Breathers
Sharks that ventilate via buccal pumping have been witnessed resting on the substrate for significant periods of time. Like humans they appear to rest by remaining very still and supporting their bodies on something. For instance, greater spotted catsharks (Scyliorhinus stellaris) appear to sleep by half-closing their eyes and resting their head against a rock (Kelly et al, 2019; Kelly et al, 2021).
Also like humans, sharks seem to be somewhat unresponsive as they sleep. For example, nurse sharks (Ginglymostoma cirratum) rest for extended periods resting in caves and they often do not react to divers at all as they slumber (Kelly et al, 2019; Kelly, 2020)
Nurse sharks are "nocturnal", meaning that their "circadian rhythm" is the opposite of (most!) humans - they rest during the day and are active at night. Several other species of shark follow the same daily pattern, including the swell shark (Cephaloscyllium ventriosum) and the horn shark (Heterodontus francisci) (Kelly et al, 2019; Kelly et al, 2021).
Yet there are also many species that rest during the day and then become active during dawn and/or dusk (known as "crepuscular"). Such as the Port Jackson shark (Heterdontus portjacksoni), the lemon shark (Negaprion brevirostris) and and whitetip reef shark (Triaenodon obesus), to name a few (Kelly et al, 2019; Kelly, 2020).
On the Move
But how do ram ventilating sharks sleep if they have to keep moving? The answer is, we are not completely sure! In some species, there is no clearly defined circadian rhythm. This suggests they either sleep in short bursts or, if they sleep over extended periods they continue swimming throughout (Kelly et al, 2019; Kelly, 2020).
Yet there are are several ram ventilating sharks with a defined circadian pattern. For example, bull sharks (Carcharhinus leucas) and blue sharks (Prionace glauca) rest in caves for extended periods. They slumber so deeply that they only respond when they are handled very roughly (Kelly et al, 2019).
Similarly, aptly-named basking sharks (Cetorhinus maximus) often bask in the sun near the surface during the day and cannot be disturbed even when they are touched (Kelly et al, 2019).
Scalloped hammerhead sharks (Sphyrna lewini) gather in large groups around a seamount, and have very low levels of activity during the day. As they sleep they will not even respond if suitable prey is around, and only rouse again to hunt during the night (Kelly et al, 2019).
No Rest
Scientists wonder if active sharks are able to sleep in a similar way to marine mammals... Some whales and dolphins are able to sleep whilst still because they only use half of their brain at a time, whilst the other half of the brain rests. This is known as "unihemispherical sleeping" (Kelly et al, 2019).
It is also possible that sharks may sleep by resting different regions of the brain one at a time. Researchers speculate that sharks may shut down the "telencephalon" (the region of the brain involved in relatively advanced cognitive tasks, like problem solving) and retain only a few 'awake' pathways which control the tail, allowing them to continue swimming whilst resting (Kelly et al, 2019).
Some scientists suggest that sharks may sleep with their whole brain, but switch to a passive method of ventilation, in order to avoid the need to keep swimming. There are several anecdotes from divers, of groups of inactive sharks resting in strong currents. Maybe these sharks selected that spot to rest because the movement of water keeps oxygen runnng over their gills whilst they are still (Kelly et al, 2019).
We will need to design new technologies and figure out how to attach a device which is capable of monitoring a shark's brainwaves underwater before we can fully understand how sharks sleep. It's exciting that there is so much yet to be discovered... But who knows... maybe sharks even dream!
References
Kelly ML, Collin SP, Hemmia JM & Lesku JA (2019). Evidence for sleep in sharks and rays: Behavioural, physiological, and evolutionary considerations. Brain Behaviour and Evolution, 94, 37–50. Access online.
Kelly LM (2020). An investigation into sleep in sharks: behavioural and electrophysiological approaches. Doctoral Thesis: University of Western Australia, Perth. Access online.
Kelly ML, Spreitzenbarth S, Kerr CC, Hemmi JM, Lesku JA, Radford CA & Collin SP (2021). Behavioural sleep in two species of buccal pumping sharks (Heterodontus portusjacksoni and Cephaloscyllium isabellum). Journal of Sleep Research, 30:3, e13139. Access online.
By Sophie A. Maycock for SharkSpeak
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