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The Parrot fish sleeps in a slimy homemade sleeping-bag

Posted in Animals, Biology, Fish, Nature, Wildlife on Wednesday, 5 June 2013

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This edited article about fish originally appeared in Look and Learn issue number 280 published on 27 May 1967.

Funny fish, picture, image, illustration

The Parrot fish changes its colour among the coral (bottom, centre)

Until about 40 years ago, the sleeping habits of fish were something nobody had ever really thought about. Zoologists knew that all animals need rest, but fish have no eyelids and as their eyes are always open, they appear to be constantly awake. Then one night, the director of the Aquarium at the London Zoo happened to go into the Aquarium. As he switched on the lights he saw at once that some fish, that by day swim in massed shoals, had separated. Each was lying motionless on its own on the base of the tank. Other fish were lying on their sides, and plaice were floating just off the bottom, instead of lying on the tank-floor, as they do by day.

In the years since then more attention has been paid to how fish spend the night, and many interesting discoveries have been made. Young soles live by day on the sea-bed. At night they come to the surface and float there fast asleep. Other fish rest on the bottom standing on their heads. But perhaps the oddest sleeping habits are found in some members of the large family known as parrot-fish. Many of these put on a kind of nightdress before going to sleep.

As night falls, each parrot-fish gives out slime from glands in its skin. During the day the fish gives out just enough slime to help it slip more easily through the water, but at night, in a way we do not yet understand, much more is given out and it forms a thin covering almost like a veil, completely surrounding the fish. This is not a careless or haphazard process, for on looking closely at the front of the veil, near the mouth of the fish, one sees that there is an opening. This is guarded by a flap that forms a valve, allowing water to enter the envelope-like case of slime. At the back of the veil there is another hole to let the water out.

These two holes mean the parrot-fish is able to draw water in through the front valve, gulp it down and pass it out through the gills for breathing. In the morning the parrot-fish breaks out of its nightgown and swims around as usual. The big mystery is why the fish should don this nightdress. One idea is that because parrot-fish live where the sea-bed is made up of fine sand, their gills might become silted up whilst they slept, and they would suffocate. Another idea is that the slimy envelope protects them from enemies whilst they are asleep and off guard.

Whatever the purpose may be, it is not something that the fish can do quickly, like slipping into or out of pyjamas or a nightdress. It takes half an hour for the fish to weave its veil, and another half an hour for it to break out of it in the morning, by biting at the slime and wriggling its body against it. It is easy to see that it must be something of a job breaking out of such unmanageable material, and one can imagine the difficulty an enemy of the fish would have breaking into it.

The mystery is deepened because not all parrot-fish put on this night-attire, and because those that do so, only use it under certain conditions. Nobody has yet discovered definitely, what those conditions are. To say the least this must be the strangest sleeping habit of any animal, and certainly one of the most puzzling.

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