Fish

Fish are aquatic vertebrates that typically live in water, breathe using gills, and move with fins. In common usage the term covers a wide range of species, but scientifically it is a broad, often paraphyletic grouping that includes both cartilaginous and bony fishes. The two largest lineages are the cartilaginous fishes (Chondrichthyes), which include sharks, rays, and chimaeras, and the bony fishes (Osteichthyes), which comprise the vast majority of species.

Most fishes have a streamlined body, scales, and a two-chambered heart adapted to an aquatic lifestyle. They exchange gases with water through gills and often possess a lateral line system to detect vibrations. Many bony fishes have a swim bladder, a gas-filled organ that helps control buoyancy. Reproduction varies: many species lay eggs with external fertilization, others have internal fertilization and produce eggs that hatch outside the body or give birth to live young. Reproductive strategies range from oviparity to ovoviviparity and viviparity, and parental care occurs in some groups.

Ecologically, fishes occupy diverse roles as predators, herbivores, and prey, and many migrate long distances between habitats to breed or feed. They form key components of aquatic ecosystems, including freshwater and marine environments, and often organize into schools or shoals for navigation and protection.

Humans rely on fish for food, income, and cultural value. Global fisheries and aquaculture provide significant protein sources but face challenges from overfishing, bycatch, habitat loss, pollution, and climate change. Sustainable management seeks to balance ecological health with economic needs.