Patricia Greene, age 9, of Emporia, Kan., for her question:
HOW CAN FISH SWIM?
Most fish gain power for forward movement in swimming by swinging the tail fin from side to side while curving the rest of the body alternately to the right and to the left.
Some fish, such as tuna and marlin, depend mainly on tail motion for thrust. Other fish, including many types of eels, rely primarily on the curving motion of the body for power.
Fish maneuver by moving their fins. To make a left turn, a fish will extend its left pectoral fin. To stop, it will extend both pectorals.
Actually, a fish's swimming ability is affected by the shape and location of its fins. Most fast, powerful swimmers, such as tuna and swordfish, have a deeply forked or crescent shaped tail fin and sickle shaped pectorals. And all of their fins are relatively large.
At the other end of the scale, most slow swimmers have a square or rounded tail fin and rounded pectorals. In this class are bullheads and bowfins.
With the exception of the very largest fish, all fish live in constant danger of being attacked and eaten by other fish and other animals. To survive, fish must be able to defend themselves against predators.
Protective coloration and protective resemblance are the most common methods of self defense, if fast escape swimming isn't possible. A fish that blends with its surroundings is more likely to escape from its enemies than one whose color or shape is very noticeabloe.
As a matter of fact, many of the fish that do not blend well with their surroundings are able to depend on their swimming speed or maneuvering ability to escape their enemies.
Like all animals, fish must rest. Many types have periods of what might be called sleep. Others simply remain inactive for short periods. But even at rest, many fish continue to move their fins to keep their position in the water.
Fish have no eyelids, and so they are unable to close their eyes when sleeping.
Even though a fish will have his eyes open while he is asleep, scientists tell us that he is probably unaware of the impressions received by his eyes at this time.
Some fish sleep on the bottom, resting on their bellies or sides. Other types sleep in midwater, in a horizontal position. The slippery dick, a coral reef fish, sleeps on the bottom and covers himself with sand.
There are more kinds of fish than all other kinds of water and land vertebrates put together. Scientists have named and described more than 21,000 kinds and each year they discover new species.
Fish first appeared on Earth about 500 million years ago, scientists tell us. They were the first animals to have backbones and may well have been the ancestors of all other vertebrates.