Ichthyology is the branch of zoology (the study of animals) that deals with fish. It includes the study of the development, anatomy (structure), physiology (function), behavior, classification, genetics, and ecology of fish, among other things. Since fish are a major food source for people, the study of ichthyology also has economic importance.
There are more than 22,000 known species of fish in the world, and they live in nearly every imaginable body of water, from stagnant ponds to the deepest oceans. They live in water all of the time and breathe through gills. Together with mammals, birds, reptiles, and amphibians, fish are one of the major groups of vertebrates (animals with a backbone). They are considered the most successful vertebrate group, outnumbering birds two to one and mammals seven to one. An ichthyologist, therefore, must contend with a great variety of subjects, from the bony, snakelike eel and the shark with all cartilage and no bone, to the bioluminescent deep-sea fish that can make its own light.
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