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A fish (pl.: fish or fishes) is an aquatic, anamniotic, gill-bearing vertebrate animal characterized by its swimming fins and hard skull, but lacking limbs with digits. Fish can be grouped into two main categories: the more basal jawless fish and the more common jawed fish, which includes not only all living cartilaginous and bony fish, but also the extinct placoderms and acanthodians. Unlike earlier classifications that grouped all fish into a single class known as Pisces, contemporary phylogenetics regards fish as a paraphyletic group, as it does not include tetrapods, which evolved from fish.

Most fish are ectothermic (cold-blooded), allowing their body temperature to fluctuate with their surroundings. However, some species, such as the white shark and tuna, possess adaptations that enable them to maintain a higher core temperature. In addition to physiological variations, many fish species have developed complex communication methods, including acoustic signals, particularly during courtship behaviors, while some fish can also produce bioluminescent displays. The scientific study of fish is known as ichthyology.

Fish originated around 530 million years ago during the Cambrian period, where they initially appeared as small, filter-feeding organisms. Over time, fish diversified into a vast array of forms during the Paleozoic era. One of the earliest known forms, the ostracoderms, featured dedicated respiratory gills and paired fins, protected by heavy bony plates functioning as exoskeletons against predators. The evolution of jaws occurred in the Silurian period with the appearance of placoderms, leading to a significant diversification during the Devonian, often referred to as the 'Age of Fishes'.

Bony fish eventually became the dominant group, as they possess key features like swim bladders and ossified endoskeletons. These are further classified into lobe-finned and ray-finned fish, the latter encompassing about 96% of all living fish species today, known as teleosts. It is fascinating to note that tetrapods, which include amphibians, reptiles, birds, and mammals, have evolutionary roots in lobe-finned fish, developing air-breathing lungs homologous to swim bladders during the Carboniferous period. Despite this evolutionary linkage, tetrapods are generally not classified as fish.

Throughout history, fish have served as a crucial resource for human societies, prominently utilized as a food source since prehistoric times. Both commercial operations and subsistence fishers harvest fish from wild fisheries or cultivate them in aquaculture systems. Additionally, fish are popular in recreational fishing and are often maintained in aquariums for aesthetic purposes. Fish have deep cultural significance, having been depicted in various art forms, religious practices, and literature as symbols of deities or other important concepts.

Etymology

The term 'fish' is derived from Proto-Germanic roots and shares connections with words like German 'Fisch', Latin 'piscis', and Old Irish 'īasc'. The exact origin remains uncertain, though some linguists propose a Proto-Indo-European root, possibly *peysk-, which is attested primarily within the Italic, Celtic, and Germanic language families.