Out of the Mouths of Babes

Do sharks shut their eyes when they sleep?

This was a question asked during a shark class I taught to some homeschooling friends (with a little help from an educational trunk borrowed from the Los Angeles Natural History Museum, and if you live in the area, I highly recommend becoming a member and taking advantage of their rental program). Olivia knows too much about sharks to have any boring, ordinary questions about them. She shot straight to something I didn't know.

Well, I sort of knew. I know that some sharks have no eyelids at all, so that would rule out the possibility of their shutting them. But that's just the start. What about the sharks that can blink? What do they do when they snooze?

I promised I would do some research and get back to her. I poked around among my shelves and did a bit of googling, and this is what I came up with.

It was once believed that all sharks had to keep moving, or they would literally drown. For most sharks this is true -- their constant movement through the water keeps the water rushing over their gill slits, which is how they breathe. Kind of like if you had to keep running with your mouth open to let the air pour in, instead of being able to just inhale and suck the oxygen you need right into your lungs.

Now we know that some sharks, such as the "flat" ones who camouflage themselves as sand or seaweed and lie on the ocean floor waiting for prey, are able to pump water over their gills instead of continually swimming. These sharks might be able to sleep as easily as any other animal. This seems to be confirmed by the behavior of nurse sharks, who possess spiracles behind their eyes that force water across their gills and who are often seen resting motionless in underwater caves.

But rest of some sort may be possible even for those sharks who must stay in perpetual motion. Sharks do not have to "think" about swimming in order to do it. The part of their nervous system that's in charge of coordinating their swimming movements is located in their spinal cords, not their brains. So even an unconscious shark can swim.

We know that dolphins shut down part of their brains at a time to sleep, and it's possible that sharks do something like this as well. We simply don't know for sure. It's surprising how little we know even about our own sleep behavior and needs, and it's that much harder to study the habits of sharks, since they generally don't do well in captivity and it's not as if we can ask them if they had pleasant dreams last night.

Whatever shark "sleep" is like, it doesn't seem to be as deep or thorough as human slumber. The eyes of an immobile nurse shark follow the movements of divers in the same cave, so even during their restful periods they're certainly not completely conked out.

So, getting back to the original question: as we don't know when, whether, or for how long sharks sleep, it seems impossible to tell if they shut their eyes or not while doing it.

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