Carolyn Ramirez, age 10, of Visalia, Calif., for her question:
HOW MANY KINDS OF FISH ARE THERE?
A scientist who studies fish is called an ichthyologist. These scientists have divided fish into two groups: jawed and jawless. Most fish have jaws, although a number of them, including lampreys and hagfish, do not have jaws. Fish are then further divided into those that have skeletons composed of bone and those that have skeletons composed of an elastic substance called cartilage.
Ichthyologists, the scientists who study fish, tell us there are more than 21,000 different kinds of fish living on earth today. Each year, however, new species are discovered, so the total number promises to increase in the future.
About 20,000 species, or about 95 percent of all the known kinds of fish, are classified as modern bony fish. This type is called teleost, coming from two Greek words meaning complete and bone.
Almost all acquarium, food and game fish are teleosts. Included are such groups as trout, tuna, perch, minnows, herring, cod, catfish and bass.
Millions of years ago, there were only a few species of teleosts. They have since become the most numerous, varied and widespread of all fish. The reason there are so many types is because they were better able than other fish to adjust to changes in their environment.
In adjusting and adapting to the changes, the bodies and internal organs of the teleosts changed in various ways. These changes are called adaptations.
About 50 species are classified as primitive bony fish. These make up about one percent of all fish species and include odd looking creatures that are related to fish that lived many millions of years ago.
Primitive bony fish include gars, lungfish, paddlefish and some sturgeon.
Sharks, rays and chimaeras total about 600 different species and make up about three percent of all known fish. All have jaws and a skeleton of cartilage rather than bone.
Almost all sharks and rays live in salt water.
The most primitive of all fish are the lampreys and hagfish. There are only about 30 species of lampreys and about 15 kinds of hagfish.
Lampreys and hagfish are slimy, scaleless creatures shaped somewhat like eels, which are teleosts. Like sharks and rays, they have skeletons made of cartilage.
Fish live almost anywhere there is water. You'll find them thriving in the icy waters of the polar regions and in the steamy lagoons of the South Pacific. Some are happy in the deepest parts of the world's seas, while others flourish in mile high mountain streams.