Surprisingly little is known about how sharks sleep

(ORDO NEWS) — Marine elasmobranch sleep expert Michael Liam Kelly talks about new research into this intriguing mystery.

Sharks used to occupy an important place in my nightmares: they followed me in the ocean, rivers, pools. But after spending some time with these elusive creatures in 2015, I began to worry about a more pressing question: Could the very creatures that invade my dreams be sleeping themselves?

As the world’s leading – and only – sleep expert on elasmobranchs (sharks and rays), my research team and I have begun to unravel this mystery, and our latest physiological evidence for sleep in sharks is the strongest to date.

Circadian rhythms

Rest, or inactivity, is often the most basic behavioral characteristic we associate with sleep. This is exactly the behavior my team was trying to identify when we started our study of sleep in sharks.

In particular, we studied the presence of circadian activity patterns, since sleep in many animals is controlled by the circadian clock (an internal biochemical oscillator).

However, sharks are a unique group of vertebrates in that many species swim continuously by passively pumping oxygen-rich seawater through their gills—they are known as ram fans. Other species manually pump seawater over their gills while remaining motionless (buccal pumps).

A study we conducted in 2020 showed the presence of diurnal activity in all studied species, both in buccal pumps and ram fans.

Importantly, in buccal pumps, these processes were internally regulated (circadian in nature). This was an important discovery and a big step in the right direction, but are periods of inactivity indicative of sleep?

Sleep or rest?

When an animal sleeps, its response and awareness of external stimulation is reduced due to sensory shutdown, or fading. As sleep researchers, we can use this ubiquitous feature of sleep to behaviorally distinguish sleep from restful rest.

Our 2021 study found that buccal pump sharks are less responsive to weak electrical impulses after five minutes of inactivity. This became the criterion for our working definition of sleep in these animals.

Sleep is also internally regulated, so animals can regain lost sleep by getting more sleep. This feature was absent in the sharks in our study – they did not make up for sleep after periods of sleep deprivation. This phenomenon is also absent during sleep in other marine fish.

These somewhat conflicting results highlight an important point: behavior can be deceptive and misleading. Animals can appear to be sleeping but still awake, and vice versa. Unfortunately, behavior alone is often not enough to reliably determine sleep in animals.

The physiology of sleep in sharks

To definitively validate our working definition of sleep in buccal pump sharks (more than five minutes of inactivity is sleep), my team set out to find physiological evidence for sleep that matched what we observed in behavior.

To do this, we recorded changes in the New Zealand checkered shark’s metabolic rate by recording oxygen consumption over a 24-hour period. A decrease in metabolic rate during sleep has been documented in many animals and is considered a reliable physiological indicator of sleep.

We also recorded subtle sleep-related behaviors in other animals, such as eye position (open/closed) and body position (vertical/flat). We found that there was no significant difference in metabolic rate between swimming sharks and sharks in a dormant state that lasted less than five minutes.

However, when the sharks were inactive for five minutes or longer, their metabolic rate dropped dramatically. This physiological change was also accompanied by a marked change in posture, with sharks moving from an upright position (sitting on their pectoral fins) to a fully recumbent position.

The condition of the eyes, however, was found to be unrelated to the state of mind of the sharks, as the animals were often observed sleeping with their eyes open.

Taken together, these data provide the most compelling evidence for the existence of sleep in sharks and support our previous behavioral findings.

Moving on

Sleep has been found in every animal studied to date, starting with flatworms and jellyfish. Sharks, as the earliest living jawed vertebrates, play an important role in understanding the evolutionary history of sleep in vertebrates.

Our research has come a long way in uncovering the previously unanswered question about sleep in sharks, but we’ve only touched the tip of the iceberg. Now that we know that (at least some) sharks do sleep, the next question to be answered is how they sleep.

Nothing is known about sleep in ram-gill sharks. Their need for constant swimming to facilitate gas exchange suggests that they may have evolved interesting adaptations to allow them to sleep in this unusual lifestyle.

Our group is now conducting electrophysiological studies of brain activity that will provide a complete picture of the form sleep takes in these animals.

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