Dennis Bolles, age 11, of Santa Cruz, Calif., for his question:
All fish, except the very largest ones, live in constant danger of being attacked and eaten by other fish or 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.
A fish that blends with its surroundings is more likely to escape from its enemies than one whose color or shape is extremely noticeable.
Many fish that do not blend with their surroundings depend on swimming speed or maneuvering ability to escape from their enemies.
If a fish species is expected to survive, the fish must know how to protect itself. If a species loses more individuals each generation than it gains, it will in time die out.
Fish also have other kinds of defenses, other than coloration and swimming speed.
Some fish, such as gars, pipefish and sea horses, are protected by a covering of thick, heavy scales or bony plates. Other species have sharp spines that are difficult for predators to swallow.
In many of these species, including scorpion fish, sting rays and stonefish, one or more of the spines are poisonous.
When threatened, the porcupine fish inflates its spine covered body with air or water until it is shaped like a balloon. The fish's larger size and erect spines may discourage an enemy.
Many eels that live on the bottom dig holes in which they hide from their enemies. Razor fish dive into sand on the bottom. A few fish do the opposite. A flying fish, for example, and the needlefish escape danger by propelling themselves out of the water.
A fish's swimming ability is affected by the shape and location of its fins. Most fast, powerful swimmers have a deeply forked or crescent shaped tail fin and sickle shaped pectorals.
Among the very fastest swimmers are the swordfish and tuna.
Most fish gain thrust or power for forward movement by swinging the tail fin from side to side while curving the rest of the body alternatively to the left and to the right. Some fish, such as the marlin and tuna, depend mainly on tail motion for thrust.
Other fish, including many kinds of eels, rely chiefly on the curving motion of the body for swimming.
Fish maneuver by moving their fins. To make a left turn, for example, a fish extends its left pectoral fin. To stop, it extends both pectorals.
Most fish are carnivores, or meat eaters. They eat shellfish, worms and other kinds of water animals. And, of course and above all, they eat other fish. They sometimes even eat their own young.
Some fish are mainly herbivores, or plant eaters. They chiefly eat algae and other water plants. But most plant eating fish probably also eat animals.