Fish are cold-blooded vertebrates living in the water.
There are cartilaginous fish (Chondrichthyes) and bony fish (Osteichthyes).
Sharks, rays and sturgeons are cartilaginous fish. Their skeleton is composed of cartilage. The skeleton of bony fish consists of bones and partially cartilage.
More than half of all living vertebrate species belong to the fish. All species of fish that live in the Central European waters are part of the real bone fish. The sturgeons species belong to the bone fishes in the broader sense.