Lisa Christensen, age 7, of Pierre, S.D., for her question:
DO FISH SLEEP?
Like all animals, fish must have rest. Most species have periods of rest that can be called sleep. Others simply stay inactive for short periods of time.
Even when they sleep, most fish must continue to move their fins to keep their position in water. And since they have no eyelids, they are unable to close their eyes when sleeping.
When a fish is asleep, it is almost certainly unaware of the impressions received by its eyes.
Some fish sleep on the bottom, resting on their bellies or sides. One species, a slippery coral reef fish, climbs under a covering of sand on the bottom when he sleeps. But most fish continue to float in a horizontal position.
Fish, like all vertebrates, have senses to tell them what is happening in their environments. They are able to see, hear, smell, taste and touch. In addition, they have a special organ called the lateral line system which enables them to "touch" objects at a distance.
Most fish have well developed senses of touch. Nerve endings throughout the skin react to the slightest pressure and change of temperature. The lateral line system senses changes in the movement of water.
Fish do not need to be shielded from sunlight because sunlight is seldom extremely bright underwater, and therefore fish lack eyelids. But they have very good eyesight. Most fish are also able to see to the right and to the left simultaneously.
A few fish are born blind. Most of these are fish that live in the total darkness of caves or in ocean depths.
Fish can hear sounds in the water and also sounds made on shore or above the water, if they are loud enough. Fish have no outer ears or eardrums to receive sound vibrations, but sound vibrations are carried to the inner ears by the body tissues.
Every fish has a sense of smell and it is highly developed in some species, including catfish, salmon and sharks.
Most fish have taste buds in various parts of the mouth, although some species also have them on other parts of the body. Catfish, sturgeon and a number of other fish have whiskerlike feelers called barbels near the mouth. They use the barbels both to taste and to touch.
Fish have inner ears which help them keep their balance. They contain a fluid and several hard, free moving ear stones called otoliths.
Whenever a fish starts to swim in other than an upright, level position, the fluid and otoliths move over sensitive nerve endings in the ears. The nerves signal the brain about the changes in body position and messages go back to the fin muscles which move to restore the fish's balance.