How Do Sharks Swim When Asleep?
In response to a question about whether or not sharks sleep and, if they do, how do they swim while unconscious, I wrote:
Sleep is one of the last bastions of mystery that Science has yet to conquer. Despite several centuries of study, speculation, and debate, we still really don't know what sleep is or why we need to 'shut down' periodically. We know that all sorts of vertebrate animals – mammals, birds, reptiles, and at least some fishes – periodically enter a period of torpor characterized by a profound change in brain waves.
But when it comes to whether or not sharks sleep, we just don't know. There have been reports from all over the globe, including Mexico, Japan, and Australia, of sharks resting motionless in caves. These have been called "sleeping sharks", but I can attest that the eyes of these quiescent sharks do follow divers moving about in the caves with them, so they're definitely not asleep.
In his sweeping epic Paradise Lost, 17th Century English poet, John Milton, asked, "What hath night to do with sleep?" We know from sonic telemetry studies (the underwater equivalent of radio tracking) that at least some shark species — such as the Reef Whitetip (Triaenodon obesus) and the Tiger Shark (Galeocerdo cuvier) — are actually more active at night than during the day. Since schooling behavior is partially coordinated by vision, many other sharks are most active at dawn and dusk, when such prey is at a disadvantage. Studies such as these suggest that at least some sharks do not sleep at night.
We used to think that the Great White Shark (Carcharodon carcharias) was primarily active during the day. But it turns out that this is largely due to observer bias: humans tend to be diurnal rather than nocturnal, and it's very difficult to watch an underwater creature at night. Recent experiments by Peter Klimley using stationary telemetry pods placed along the bottom at the Farallon Islands, we have learned that White Sharks are quite active at night, when they probably feed primarily on bottom fishes. If these bottom fishes are sleeping when attacked, they're certainly in for a 'rude awakening'!
Because sharks rely on ram-ventilation — that is, their forward swimming to push oxygen-bearing water through their mouths and over their gills — this fact begs the question: do sharks sleep at all? Dolphins are conscious breathers, actually having to think about rising to the surface, opening their blowholes, and breathing. From Russian studies carried out in the early 1970's, we know that dolphins catnap in 2 to 3-minute stretches, actually shutting down one hemisphere of their brain at a time. From experiments carried out on a small shark called the Spiny Dogfish (Squalus acanthias), we know that the 'Central Pattern Generator' that co-ordinates swimming movements in sharks is not located in the brain, but in the spinal chord. Thus, it is possible for an unconscious shark to swim.
There is even a possibility that the sharks' fore, mid-, and hind- brain shut down in sequence, resulting in the shark equivalent of sleep-walking.
I hope this answers your question.
— R. Aidan Martin