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General Science => Question of the Week => Topic started by: thedoc on 15/04/2009 13:47:06

Title: QotW - 09.04.19 - Why don't fish have eyelids?
Post by: thedoc on 15/04/2009 13:47:06

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?
Asked by Lauren, California

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Title: Why don't fish have eyelids?
Post by: thedoc on 15/04/2009 13:47:06

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

[img float=right]http://www.thenakedscientists.com/forum/copies/RTEmagicC_445px-Salon_agriculture_2009_-_Poissons.jpg.jpg[/img]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 the 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.

Title: QotW - 09.04.19 - Why don't fish have eyelids?
Post by: Chemistry4me on 15/04/2009 13:56:07

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'. [:0]

Title: QotW - 09.04.19 - Why don't fish have eyelids?
Post by: nubemet on 20/04/2009 09:29:43

Very Interesting.
I still don't know what do I need those eyelids for :)

Title: QotW - 09.04.19 - Why don't fish have eyelids?
Post by: Chemistry4me on 21/04/2009 02:08:30

To moisten your eyeballs fella!

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Post by: jenni on 23/02/2010 21:36:41

ooohh wow u r soo stupid he just said dat!!!!

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Post by: lallah on 31/03/2010 03:12:10

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

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Post by: jbs on 07/10/2014 05:37:26

Eye lids also modulate light entry to eye ball in animals with linear vertically oriented pupils, like cats.

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Post by: pip on 04/03/2015 21:50:52

You're angry one jenni.