Fish Diversity

Fish — the most species-rich group of vertebrates, comprising approximately 33,000 described species — occupy virtually every aquatic habitat from Arctic sea ice to deep ocean hydrothermal vents, from mountain streams to hypersaline lakes. They include the jawless fish (lampreys, hagfish), cartilaginous fish (sharks, rays), and ray-finned fish (which includes most familiar fish). Fish were the first vertebrates — the evolutionary ancestors of all land vertebrates, including humans. Extremes: the Paedocypris fish (6-8mm, Sumatra) is the world's smallest vertebrate; the whale shark (18m) is the world's largest fish; the ocean sunfish (Mola mola) is the world's heaviest bony fish (2,300 kg); the oarfish (8-11m) is the world's longest bony fish and the likely inspiration for sea serpent myths. The coelacanth — a 'living fossil' fish thought extinct for 65 million years — was discovered alive in 1938 off South Africa, the most dramatic zoological discovery of the 20th century.