What Are the Characteristics of Fish?

A fish is scientifically classified as any aquatic vertebrate with gills and lacks limbs with fingers, toes or other digits. Whether in high mountain lakes, country streams, rivers or the deepest oceans, these cold-blooded animals are present in almost any natural body of water. Though fish are a very diverse group of creatures with more than 32,000 presently existing species, several key characteristics link the majority of them together.

  1. Gills

    • Like mammals using their lungs to take in oxygen on land, a fish uses its gills to ingest dissolved oxygen from the water and expel carbon dioxide afterward. The gills of vertebrates are usually located along the exterior of the pharynx, while the gills of cartilaginous fish open directly to the outside of the body. Bony fish have five pairs of gills, which are contained in a brochial chamber, and hagfish have spherelike gills, which utilize circular openings to garner oxygen.

    Cold Blood

    • Fish are ectotherms: creatures that adjust or control their body temperature through their external environment. Most fish belong to a subtype called homeotherms: cold-blooded creatures that live in water that stays a consistent temperature. This classification gives fish two advantages -- the ability to maintain their regular body temperature in extreme weather and the ability to specialize in certain enzymes specific to creatures that live at constant temperatures.

    Senses

    • Fish use a combination of their senses of sight, smell, taste, hearing and touch to locate food, find a mate and avoid predators. Many fish can see color and perceive various levels of light, using spherical eye lenses to focus on specific targets. Using their keen senses of taste and smell to detect nearby chemicals, predators and prey, fish also use internal ears to help with balance and pick up on audible vibrations.

    Schooling

    • Up to 20 percent of adult fish and 80 percent of young fish school together, either swimming in near synchronicity or swimming in less-organized groups. Scientists theorize that fish school to reduce the chances of being eaten, to increase their ability to forage for food and to save energy by pulling each other upstream.