Dawn Argenziano, age 11, of Staten Island, N.Y., for her question:

HOW MANY KINDS OF FISH ARE THERE?

A fish is a vertebrate that lives in water. A   vertebrate is a backboned animal. There are more different kinds of fish living in the world than all other kinds of water and land vertebrates put together.

We know there are at least 21,000 different kinds of fish on earth. New species are discovered each year, so the total is constantly increasing.

Most fish are grouped as jawed species. There are also some classified as jawless. The jawed fish are divided into two further groups: bony fish and cartilage fish.

Cartilage fish have skeletons composed of a tough, elastic material. In this group are the sharks, rays and chimaeras. About 95 percent of all fish are classified as bony because of their skeletons.