What Characteristics Do All Fish Have in Common?

All fish are cold-blooded, have vertebrae, have fins and breathe through gills. Most fish have skeletons made of bone though sharks and rays have skeletons made of cartilage.

Fish also have a head, body and tail. Other than this, the size, shape and other characteristics of fish vary widely.

The heads of fish contain the eyes, nostrils, gills and mouth. With the exception of hagfish and lampreys, fish also have upper and lower jaws. The position of a fish's mouth can be used to determine its species. The mouth is terminal if it's at the tip of the head and inferior if it's underneath the head. It's superior if the lower jaw juts out past the upper, and it's subterminal if the upper jaw juts out past the lower jaw.

Fish can also have teeth mouths and throats as well as in their jaws. Some fish have large conical teeth, while others have tiny teeth arranged in masses, or villiform teeth. Some fish have canine teeth, while others have teeth that resemble incisors or molars. Some species, such as the wolf fish, have multiple types of teeth.

Most fish have scales, though some lack them. Scales are often used to identify the species of a fish.