A fish is an aquatic animal that breathes through gills. There are more than 30,000 species of fish in the oceans and the freshwater streams. Some have a boney skeleton, while others, like the shark, have older origins and have a cartilaginous skeleton instead. To move around in the aquatic environment, fish will use appendages that have evolved for swimming: their fins. Some will also use their hydrodynamic, elongated bodies to propel themselves.