Fish are water-dwelling vertebrates. They move in the water by using their fins. They filter oxygen from the water with their gills.

Fish reproduce by spawning. During spawning, a female fish lays down its spawn (egg cells), and the male fish fertilizes them with its milt (sperm cells). This means that fish reproduce sexually.

Fish are poikilothermic animals. This means that their body temperature changes depending on the temperature of the water around them. As a result, fish are rather inactive during the cold winter months. 

Fish have simple circulatory systems. The heart of a fish contains only one atrium and one ventricle, whereas the heart of mammals contains two of each. 

Fishes can be divided into cartilagenous and bony fishes, depending on the material from which their skeletons are made out of. Most fish species are bony fishes, whereas sharks are examples of cartilagenous fishes.