#1207 Do fish sleep?

Do fish sleep? All fish sleep, but they have different ways of doing it.

Almost all complex organisms, with a few exceptions, need to sleep, although they all have different sleep requirements and methods. Ants microsleep for a few minutes at a time, many times a day, while dolphins put one side of their brain to sleep at a time. Dolphins don’t breathe unconsciously, as we do, and if they lose consciousness to sleep, they would drown.

Nobody really knows why we need to sleep. However, as it is ubiquitous across nearly all lifeforms, and as we can see what happens when lifeforms are deprived of sleep, we can tell that it is vital. The exact reasons are unknown, but it does clean out the brain and allows memories to be laid down, among other things.

So, how do fish sleep? We sleep by closing our eyes, lying down, and shutting down the neocortex part of our brain. The neocortex is responsible for many things, but some of them are language processing, social and emotional processing, and memory. Shutting this down gives some weight to the theory that all of these things are stored by the neocortex during sleep. Well, fish don’t have eyelids, so they can’t close their eyes, and they don’t have a neocortex to shut down. They also don’t have a safe warm bed to lie down in. Fish have to cope with ocean currents, the fact that things generally sink in water, and predators. Some fish also have to cope with the fact that they need to keep moving to breathe.

The majority of fish sleep by slowing down their metabolisms in the same way that we do. Their heart rate goes down and their breathing slows down as well. They become motionless and a lot of them just float. Smaller fish will usually dig a trench at the bottom of the water and hide there, sleep among a lot of other fish, or hide under leaves or in caves. Most of the research done on sleeping in fish has been carried out in aquariums, which don’t offer all of the same conditions that fish would be faced with in the wild. There are no strong currents for one, and all of the fish have been selected so they don’t eat each other.

Fish have different sleep requirements as well. Some sleep on and off throughout the day and some sleep once it gets dark. There are several nocturnal fish that have evolved to eat sleeping fish and a lot of fish vary their sleep patterns or positions to avoid them.

Sharks are an interesting exception to this. Some sharks do sleep and some only sleep with one side of their brain at a time. Sharks have three different ways of breathing and it varies by species. Some sharks are buccal pumpers, but they are a minority. That means they use their cheek muscles to pull water in through their mouths and force it out through their gills. The oxygen in the water gets absorbed by the gills and the sharks can breathe. Because they are forcing the water through themselves, they can breathe when they are stationary. Examples of buccal pumping sharks are nurse sharks and leopard sharks. The second type of shark are obligate ram ventilators. In the same way as a ramjet engine (although it forces air and not water), they ram water over their gills by using forward momentum. Some examples of obligate ram ventilator sharks are whale sharks, great white sharks, hammerhead sharks, and mako sharks. Because the only way the oxygen reaches their gills is through the shark’s forward momentum, they have to constantly keep moving or they suffocate. The last type of sark can switch between the different breathing methods. Reef sharks can do this. When it comes to sleeping, buccal pumping sharks can sleep anywhere in the same way as fish do. They float in the water and continue to breathe while they sleep. However, obligate ram ventilators can’t stop and sleep because they would suffocate. They have to keep moving, which means they only sleep with one side of their brain at a time. Researchers have discovered that some sharks can completely sleep if they position themselves against a strong ocean current. If they can float with their nose pointing into a strong current, the water will be forced over their gills and they will be able to sleep. However, they have to stay alert in case the current stops. And this is what I learned today.

Sources

https://www.reefcentral.com/forums/showthread.php?t=2610032

https://oceanservice.noaa.gov/facts/fish-sleep.html

https://www.aqueon.com/articles/fish-sleeping-habits

https://en.wikipedia.org/wiki/Neocortex