Knowledge Expansion The main characteristics of fish are that they have spines, fins and mainly use fins to move, real upper and lower jaws, and use gills to breathe. Whales are called fish, but they are not fish, because they breathe with their lungs. Similarly, octopus is also called fish, but it is not a fish, but a mollusk. Because they have no spine and no fins. Although the seahorse does not have hairtail in its name, it is indeed a kind of fish, because it completely conforms to all the characteristics of fish.
