Why don't fish have eyelids?

19 April 2009



Why do fish not have eyelids? My goldfish and coy fish assume a nightly ritual of gathering at one end of the tank when I turn out the lights. They're still there in the morning. How long do they sleep and how can they rest if their eyes never close?


We put this to Mark Briffa, Marine Behavioural Ecologist, University of Plymouth

With the first part of this question we really have to go back into the evolutionary history of the vertebrate eye which is the same basic structure in fish and humans.

The eye first evolved in the ancestors of fish. It's well-suited to an aquatic environment where its surface is kept moist all the time. They eye isn't so well-suited to exposure to the air where it could dry out. The reason fish don't have eyelids is because, underwater, they don't need eyelids.

Not having an eyelid doesn't mean that the fish can't go to sleep. I'm sure that many listeners have experienced falling asleep with the radio on, hopefully not during the Naked Scientists.

The reason you can do this is because sleep isn't a complete lack of awareness of what's going on in your surroundings. It's a period of reduced responses to external stimuli.

Not having eyelids to completely shut out the light doesn't mean that fish can't sleep and a recent study on zebra fish - a common fish kept as pets - shows that these fish can fall asleep, and when they're sleeping they sink to the bottom of the tank and their tails kind of droop down as they become more relaxed. Some zebra fish apparently even experience insomnia.

Learning how long and how often fish can sleep for (and it's quite a new area) could help us with understanding sleep problems in humans.

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Why don’t fish have titties

Fish are not mammals, which are the subset of animals that produce milk - delivered via mammary glands - with which they feed their live young. Fish lay eggs and the hatchlings are largely capable of being indepedent from birth, so they find their own food.

I was just looking at the fish in our tank. They are varius species of goldfish. One in particular has bulginmg "googly" eyes. This led me to wonder if they have eyelids since I cannot see the fish blink. So, this was a great explanation for why they don't need eyelids. Thank you very much for this simple, yet interesting response.

Why dont they have eyelashes

It is interesting to know about them

That is AMAZING!!!!!!!!!!!!!!

i like fish

This is quite fascinating information.

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