I have to confess that I have been spending way too much time binge watching shows about sharks during this year’s Shark Week on Discovery Channel. I know I should be getting 7 hours of sleep a night…but could watching just one more show be that bad?
Speaking of sleep, I am reminded of a study that I read awhile back examining whether sharks and rays need sleep. This may seem like a silly question, but most sleep research has focused on animals that live on land probably because the question is easier to study in terrestrial animals than aquatic species. A new study published in Brain, Behavior and Evolution attempts to fill in the missing pieces by presenting information they found on sleep in cartilaginous and bony fishes. Although they found scant data for sleep in sharks and rays, they surprisingly found no data for sleep in continuously swimming fishes.
Sleep is associated with characteristic changes in brain activity. Both mammals and birds are known to experience multiple sleep states including REM (rapid eye movement) and non-REM states. Some studies suggest that reptiles, zebrafish, cuttlefish and even flies may similarly experience at least two states of sleep, but the evidence is not clear how similar these states are to what birds and mammals experience. In case you were wondering, you can tell a zebrafish is sleeping when they hang out near the top or bottom of the tank and no longer swim (and are still alive). Studies suggest that sleep-deprived zebrafish, like people, will sleep longer if they are kept awake too long prior to the sleep bout. In contrast, it appears that cavefish do not need to sleep as long as their relatives who live outside of caves.
Okay, back to sharks. For gas exchange to occur, sharks need water to move continuously across their gills. Some sharks have spiracles behind their eyes, which are small openings that move water across their gills even when they are lying still. You can imagine that some have speculated the animals could not survive if they stopped swimming whereas some studies suggest that the animals may actually sleep. In fact, some species of sharks never seem to stop moving, whereas others do seem to rest for prolonged periods of time. For example, one study showed that a captive nurse shark, Ginglymostoma cirratum, is mainly active at night. That means, they are not very responsive during the day to disturbances, like scuba divers. But I would still avoid confronting one. Similarly, many other species of sharks appear to be nocturnal and may spend quite a bit of time snoozing. In contrast, common stingrays have reduced motor activities at night. During periods of apparent wakefulness, the animals appear to move more and have a higher metabolic rate.
Similar to some species of sharks, cetaceans such as whales, dolphins, and porpoises continuously swim and do not stop to sleep. Instead, they only rest half of their brain at a time. This is called unihemispheric sleep. Some seals sleep like cetaceans while in the water but switch to bihemispheric sleep (using both hemispheres – like people) on land. Continuously swimming sharks may also experience unihemispheric sleep. An advantage to unihemispheric sleep is that with only half the brain asleep, the other half can pay attention to obstacles and predators and keep an eye out for a tasty meal.
All this talk of sleep has made me a bit tired too. Goodnight!
Kelly ML, Collin SP, Hemmi JM, Lesku JA. Evidence for sleep in sharks and rays: Behavioural, Physiological, and Evolutionary Considerations. Brain, Behavior and Evolution. 94: 37-50, 2019.