Question of the Week

Why don't fish have eyelids?

Sun, 19th Apr 2009

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Lauren, California asked:

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. Itís well-suited to an aquatic environment where its surface is kept moist all the time. They eye isnít so well-suited to exposure to the air where it could dry out. The reason fish donít have eyelids is because underwater they donít need eyelids. Not having an eyelid doesnít mean that the fish canít go to sleep. Iím 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 isnít a complete lack of awareness of whatís going on in your surroundings. Itís a period of reduced responses to external stimuli. Not having eyelids to completely shut out the light doesnít mean that fish canít sleep and the recent study on zebra fish Ė a common fish kept as pets shows that these fish can fall asleep and when theyíre 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 itís quite a new area) could help us with understanding sleep problems in humans.


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Humans have eyelids to prevent the cornea drying out, we blink to moisten them. Fish do not need eyelids as their eyes are constantly moist.
Their 'sleep' is probably different to most land animals, they reduce activity and metabolism which is probably their version of sleep. Fish do not show brainwave patterns like REM sleep seen in humans. I think some sharks have to swim while 'sleeping'. Chemistry4me, Wed, 15th Apr 2009

Very Interesting.
I still don't know what do I need those eyelids for :) nubemet, Mon, 20th Apr 2009

To moisten your eyeballs fella! Chemistry4me, Tue, 21st Apr 2009

ooohh wow u r soo stupid he just said dat!!!! jenni, Tue, 23rd Feb 2010

wow i dindnt no sharks sleep while they still swim and that fish eyes are always moist from the water that there in and that fish dont have eye lids lallah, Wed, 31st Mar 2010

Eye lids also modulate light entry to eye ball in animals with linear vertically oriented pupils, like cats. jbs, Tue, 7th Oct 2014

You're angry one jenni. pip, Wed, 4th Mar 2015

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