Andrea Bartlett, age 9, of Tacoma, Wash., for her question
HOW MANY KINDS OF FISH ARE THERE?
We now know that there are more than 21,000 different kinds of fish on earth. But each year scientists discover new species, so the total increases continually.
Fish make up more than half of all the known species of animals with backbones. Scientists who study fish are called ichthyologists. They divide the earth's fish into two main groups: jawed and jawless.
Most fish have jaws. The only jawless kinds are lampreys and hagfish.
Jawed fish are further divided into two groups: those with skeletons composed of a tough, elastic material called cartilage and those with skeletons composed largely or partly of bone.
Fish with skeletons composed largely of bone are called modern bony fish while those with skeletons partly of bone and partly of cartilage are called primitive bony fish.
More than 20,000 species of fish are in the modern bony fish group. They make up more than 95 percent of all known kinds of fish. Nearly all food, game and aquarium fish fall into this classification.
Many millions of years ago, there were only a few species of modern bony fish. They were greatly outnumbered by sharks and by the ancestors of certain present day bony fish. They became the most numerous, varied and widespread of all fish mainly because they were better able to adapt or adjust to changes in their environment.
Today, the various species of modern bony fish, also called teleosts, differ from one another in so many different ways that they seem to have little in common.
There are about 50 kinds of primitive bony fish living today, including gars, lungfish, sturgeons, bowfins and bichirs. They make up less than 1 percent of all fish species.
There are about 600 species, or 3 percent of all known fish, in the group with skeletons of cartilage. There are also about 30 species of lampreys and about 15 of hagfish.
Fish with skeletons of cartilage include sharks and rays.
Most sharks have torpedo shaped bodies while most rays are shaped somewhat like pancakes. A large, wing like fin extends outward from each side of a ray's flattened head and body.
The best way to tell a shark from a ray is by the position of the gill slits. In sharks and rays, gill slits are slot like openings on the outside of the body, leading from the gills. A shark's gill slits are on the sides of its head just back of the eyes. A ray's are underneath its side fins.
Lampreys and hagfish are the most primitive of all fish. Like sharks and rays, they have skeletons made of cartilage. But unlike all other fish, lampreys and hagfish lack jaws. A lamprey's mouth is mainly a round sucking organ and a toothed tongue. Hagfish have slit like mouths, sharp teeth but no sucking organ.