Question: what are the main characteristics of the three major groups of fishes?
Answer: Scientists group fish into three main types. They are divided into these groups because of the structure of their mouths and the types of skeletons they have. There are jawless fishes, cartilaginous (cart uhl AJ uh nuhs) fishes, and bony fishes. All three types are "cold-blooded," or ectotherms.