Anne Guthrie, of St Joseph's School, asks :-
Do fish sleep in their fish bowls at night and if so do their gills still work when they sleep?
Glen Davidson, a physiologist at Massey University, responded.
Fish do show behaviour that resembles sleeping. Since fish do not have eyelids, it is very difficult to tell, just by looking, if they are asleep or not. At certain times, fish in the home aquarium will hang motionless in the water. This is when they are "sleeping". This is especially true at night, as many fish are only active during the day. Try turning on the light in an aquarium a short time after it would have normally been switched off for the night. Many of the fish will be motionless near the bottom of the tank. Most fish continue to breath by opening and closing their mouths, but some fish, such as some sharks and tuna, cannot do this. Instead they must continue to swim to keep water flowing over their gills, even when they are "asleep".