David Hare, age 11, of Allentown, Penn., for his question:

DO FISH SLEEP?

Many species of fish live mainly by themselves. Such fish include most of the predatory fish. They join other fish only for mating. Other species live together in closely knit groups called schools. About a fifth of all fish species are schooling species. Some schools may have fewer than 25 individuals while others may number into the hundreds of millions.

Fish live almost anywhere there is water, and that's just about any place in the world. You'll find them in the near freezing waters of the Arctic and the steaming hot water of tropical jungles.

Fish are animals. And like all animals, they need rest. Many species have periods of what might

be called sleep while others simply remain inactive for short periods of time.

But even at rest or sleep, many fish continue to move their fins to keep their position in the water.

Fish do not have eyelids and are therefore unable to close their eyes when sleeping. But while they are asleep, fish are probably unaware of the impressions that are received by their eyes.

Some fish sleep on the bottom, resting on their belly or side. Other species like to sleep in midwater, in a horizontal position. One type, a coral reef fish called the slippery dick, sleeps on the bottom under a covering of sand.

Another coral reef fish, this one called the striped parrot fish, encloses himself in an envelope of mucus before going to sleep. This fish secretes the mucus from special glands in its gill chambers.

Some types of air breathing fish, such as the African and South American lungifsh, sleep out of water for months at a time. These fish live in rivers or ponds that are dry during the summer. The fish lie buried in hardened mud until the return of the rainy  seasons.

This type of long summer sleep is called estivation. During estivation, a fish breathes little and lives off the fat stored in his body.

Fish in a school usually travel in close formation as a defense against predators. But a school often breaks up at night to feed and rest or sleep, and then regroups the next morning.

Among many species of fish, including cod and perch, a number of individuals may gather in the same area for feeding, resting or spawning. Such a group is only temporary and not so closely knit as a school.

All fish, except the largest ones, live in constant danger of being attacked and eaten by other fish or other animals. So even in rest they are quickly able to move.