All fish share two traits: they live in water and they have a backbone—they are vertebrates. Apart from these similarities, however, many of the species in this group differ markedly from one. Common names of fish can refer to a single species; to an entire group of species, such as a genus or family; or to multiple unrelated species or groups. Ambiguous common names are accompanied by their possible meanings. The ultimate guide to fishes & fish life, including definition of a fish, fish bodies, types of fish and the life-cycle of a fish. Read on for a complete guide to ichthyology… Fish (plural: fish or fishes) are a group of animals which live in water and respire (get oxygen) from their gills. As a group, they are much older than other vertebrates. Fish are water animals that live in freshwater or saltwater and breathe through gills. Some fish are small and colorful, while others are large and used for food, fishing, or aquarium keeping. Salmon, tuna, goldfish, trout, shark, and catfish are some common examples of different types of fish. Instead, there are 3 main classes, groups, or types of fish: bony fish (Osteichthyes), jawless fish (Agnatha), and cartilaginous fish (Chondrichthyes). Fish are the most diverse group among vertebrates, with over 33,000 different types of fish species. Fish are vertebrates (vertebrates have backbones) that live in water. They breathe using special organs called gills. Fish have been an important natural resource for humans since prehistoric times, especially as food. Commercial and subsistence fishers harvest fish in wild fisheries or farm them in ponds or breeding cages in the ocean. This guide explores 120 types of fish, offering a clear look at their distinctive features, behaviors, habitats, and ecological roles. Whether you’re a beginner fish enthusiast or an experienced naturalist, these descriptions help you identify and appreciate the variety in the underwater world. fish, any of approximately 34,000 species of vertebrate animals (phylum Chordata) found in the fresh and salt waters of the world. Living species range from the primitive jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes.