Colleen Langdon, age 11, of Omaha, Neb., for her question:
HOW MANY DIFFERENT FISHES ARE THERE?
Imagine what a job it must be to identify all the different fishes in the enormous ocean, plus their cousins who prefer life in freshwater lakes and streams. Scientists have been trying to do this for ages, though the complete job is far from finished. So far the known fishes outnumber all the other backboned animals. But every now and then strangers are discovered, and the list grows longer.
The list of backboned animal species includes 3,000 froggy amphibians, 5,000 furry mammals and 9,000 feathery birds. Most of these vertebrates live entirely or partly on land. The fishes are vertebrates of fresh and salt water, and so far we know there are at least 21,000 different kinds.
More species are added to the list, especially when marine biologists probe down into the mysterious, midnight depths of the ocean abyss. Some experts suspect that when all the fishy species are counted, they will outnumber all the other vertebrates by 2 to 1.
In order to qualify as a fish, an animal must be a coldblooded creature with a skeleton, plus gills of some sort to extract oxygen from the water. In the past, all animals with these qualifications were classified in a large group called Pisces, meaning the Fishes. Nowadays, most American scientists arrange them into three major classes, which are subdivided into smaller groups.
One major class includes the lampreys and other jawless fishes. Another is for the sharky types, with skeletons of gristly cartilage. A third major class includes the vast assortment of ordinary bony fishes. This large group is subdivided into more than 30 orders of somewhat similar fishes.
The various orders are subdivided into related family groups. Each family is composed of one or more closely related genus groups. And each kind of fish belongs in his own species. To scientists he is known by a double name, which gives his genus and species. As a rule, his scientific names give a few clues to his identity.
For example, the ever popular Goldie the goldfish belongs in the Cypririiforme order, a name which means the carp type fishes. Her family name is Cyprinidae, and it includes other carp types. Her special name, which includes her genus and species, is Carassius auratus which means the golden carp.