Fish are also called "vertebrates" because of their skeleton and they display a wide variety of colors and sizes. They are different from other sea animals due to their scales and fins. There are over 15,000 species of vertebrates in the marine environment. The different fish families have significant differences between them. Each of their fins corresponds to a function (stability, change of direction during swimming, etc.). The senses of vertebrates are similar to those of humans, meaning they can hear, see, smell, taste and feel the movement of water through a sensory organ called the "lateral line". Do you ever wonder how they can breathe underwater? But with their gills, that capture oxygen and release carbon dioxide from their blood, of course.