Fish are a very rich and diverse group of animals extremely well adapted to life in water. On their own, they account for almost the same number of species (around 24,000) as the other groups of vertebrates (amphibians, reptiles, birds, and mammals). There are such major differences within this group that it is hard to define a general characterisation common to all fish.
There are three main groups of fish, with different body shapes and internal structures, as a result of different evolutionary histories:
• jawless fish - lampreys and hagfish - 85 species;
• cartilaginous fish - skeleton made of cartilage - 850 species;
• bony fish - bony skeleton - over 23,000 species.