water flow in over their gills. One great white has been filmed doing just that – her slow swimming and open mouth supposedly enabling her to keep oxygen-rich water moving over her gills while she rests.
While we aren’t yet sure if this is sleep swimming, we do know that sharks using only ram ventilation to breathe, such as great whites, evolved to be more active cruisers of the open ocean and lost the ability and structures to actively pump water over their gills while not moving forward.
Sharks that pump water over their gills using their cheek and neck, otherwise known as ‘buccal’ muscles, breathe by means of ‘buccal pumping’. This way of breathing was used by the ancient sharks that first evolved and cruised the oceans 450 million years ago, and many shark species still use buccal pumping today. They include Port Jackson sharks, leopard sharks and zebra sharks. Species that use buccal pumping can spend their time resting, or dare I say snoozing, on the seafloor.
Sharks that sleep all day and swim all night
In the wild and aquariums, some sharks that breathe using buccal pumping are often seen lying still on the sand during the day and actively swimming around at night. Including nursehound sharks and nurse sharks in aquariums, they spend most of their days lying still at the bottom of the tank in a relaxed posture. Nursehound sharks’ eyes are even slightly closed while they appear to be resting. Nurse sharks are less responsive when ‘resting’ and don’t react to scuba divers at all until touched by them, which makes the sharks swim away. Wild nurse sharks are typically docile and not known to be aggressive towards humans but have bitten scuba divers that startled them while they were ‘resting’. None of the bites caused fatalities and were likely done in self-defence following a fright. It seems that while these sharks were ‘resting’ they had decreased awareness of their surroundings and lacked movement that could quickly be reversed, which fits the definition of sleep, right? Well, the definition of sleep has a second part to it: if sleep is lost it leads to more sleep later, known as ‘sleep rebound’. This is the main way in which sleep differs from rest because lost rest doesn’t result in more rest later.
It’s tricky to study shark sleep in the wild because sharks are generally shy and elusive and in aquariums they don’t behave in the same way. For example, at night whitetip reef sharks swim three times more in the wild than they do in aquariums. It’s tricky to pinpoint why sharks in aquariums behave differently, but maybe it has to do with the way they’re fed or how the tank is set up. For these reasons, it’s difficult to ascertain whether sharks sleep rebound.
Snoozing in snuggle piles
Nurse sharks often spend their days resting in groups of up to as many as 40 sharks in the wild. Some appear to be cuddling and snuggling while they rest. Shark scientists think resting together may help protect them from predators, including formidable bull sharks.
Sharks, and some other species, that bury themselves in sand can breathe through spiracles. Spiracles are openings just behind the creatures’ eyes that function like snorkels. Oxygen-rich water is sucked in through these spiracles (sometimes from above the sand) and pumped through the gills and out the gill slits. Sharks that have spiracles include serrated snouted sawsharks, flattened angelsharks and furry-faced wobbegongs. These sharks could be snoozing while they lie there breathing through their snorkels, we just don’t know for sure.
Many species of shark take in oxygen-rich water through spiracles and use buccal pumping, while other more active species use buccal pumping and ram ventilation. Ragged-tooth sharks can switch between buccal pumping and ram ventilation depending on how fast they’re swimming – you can see these sharks in the Save Our Seas Foundation Shark Exhibit at the Two Oceans Aquarium in Cape Town South Africa and in many other aquariums around the world. Another species that switches between these two types of breathing is the mighty tiger shark.
Ram ventilating sharks, which usually need to keep moving to keep water moving over their gills, can even survive staying stationary if conditions are just right. Caribbean reef sharks, which typically only use ram ventilation, have been found motionless ‘resting’ in caves in Mexico, baffling shark scientists. Scientists later discovered that the water in the caves had extremely high levels of oxygen and reduced salinity. These conditions allowed the sharks to keep breathing without moving because the conditions allowed for easier absorption of oxygen from the water around them.
Other ram ventilators that travel long distances through the open ocean such as great whites, blue sharks, shortfin mako sharks and even whale sharks probably rest while ‘yoyo swimming’. What’s yoyo swimming? It’s when sharks actively swim towards the surface and then rest as they make a gliding descent back to the depths. This technique of zig-zagging up and down enables ram ventilating species to have water flowing over their gills as they rest on the way down, and to cover extremely long distances efficiently.
Some sharks have rest periods, but whether they’re really asleep while they’re resting is still a mystery.
Shark scientists define rest as ‘reduced activity without loss of consciousness or greatly reduced responsiveness’ while sleep is ‘reduced activity and a reduced state of consciousness that can be quickly reversed’. Lost sleep leads to an increased need for sleep or a ‘sleep rebound’, while lost rest doesn’t.
Some sharks definitely rest and this can at least superficially resemble sleep. However, to solve the mystery of whether sharks sleep scientists will have to work out if sharks make up for lost sleep. This is tricky because in the wild sharks are elusive and in captivity they behave differently from how they do in the wild.
Sharks don’t shut their eyes while resting, as we do, and how they rest depends on which type of breathing they use.
For sharks, breathing entails keeping oxygen-rich water moving over their gills.
Some sharks must keep swimming to keep breathing to stay alive, but others can breathe while stationary or even buried in the sand.
Despite decades of research on sharks, scientists still haven’t been able to prove if sharks sleep or not.
Do sharks sleep?
Shark scientists aren’t sure if sharks really sleep, but some have periods of deep rest that at least superficially look like sleep. To find out if sharks sleep or not shark scientists will need to work out if sharks that lose out on rest need to rest for longer the next time they rest. If they do it will mean that sharks sleep.
Can sharks shut their eyes?
Sharks have eyelids like you and me but they can’t close them all the way, if at all. Most sharks also have transparent nictitating eyelids called third eyelids that slide over their eyes just before they bite. These eyelids protect their eyes while still allowing them to see. During rest, sharks’ eyes are open to monitor things moving in their environment.
Must sharks keep swimming to stay alive?
Not all species of shark need to swim constantly to stay alive. Sharks that only use ram ventilation to breathe do need to keep swimming to keep oxygen-rich water flowing over their gills and stay alive, but species that use buccal pumping and breathe through spiracles don’t need to keep swimming to stay alive.
How do sharks breathe?
Sharks breathe by keeping oxygen-rich water moving over their gills. They extract oxygen from the water moving over their gills into their blood. There are three ways they keep water moving over their gills:
Ram ventilation - sharks swim constantly so that oxygen-rich water is continually rammed over their gills. Under most ocean conditions if they stop swimming they’ll drown.
Buccal pumping - sharks use their cheeks to pump water over their gills. This allows them to stop swimming without drowning.
Spiracles - harks have openings called spiracles behind their eyes which they use to pump water over their gills. Their spiracles essentially act as snorkels.
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