A fish is a water-dwelling vertebrate with gills that doesn't change form, as amphibians do, during its life. Most are cold-blooded, though some (such as some species of tuna and shark) are warm-blooded. There are over 29,000 species of fish, making them the most diverse group of vertebrates. Taxonomically, fish are a paraphyletic group whose exact relationships are much debated; a common division is into the jawless fish (class Agnatha, 75 species including lampreys and hagfish), the cartilaginous fish (class Chondrichthyes, 800 species including sharks and rays), with the remainder classed as bony fish (class ).

Fish come in different sizes, from the 16 m (51 ft) whale shark to a 8 mm (just over ¼ of an inch) long stout infantfish. Many types of aquatic animals named "fish" are not true fish, and in the case of animals such as jellyfish and cuttlefish, are not even vertebrates. Other marine creatures that have in the past been considered fish, like dolphins, are actually mammals.

Although most fish are exclusively aquatic and cold-blooded, there are exceptions to both cases. Fish from a number of different groups have evolved the capacity to live out of the water for extended periods of time. Of these amphibious fish some such as the mudskipper can live and move about on land for up to several days. Also, certain species of fish maintain elevated body temperatures to varying degrees. Endothermic teleosts (bony fishes) are all in the suborder Scombroidei and include the billfishes, tunas, and one species of "primitive" mackerel (Gasterochisma melampus). All sharks in the family Lamnidae – shortfin mako, long fin mako, white, porbeagle, and salmon shark – are known to have the capacity for endothermy, and evidence suggests the trait exists in family Alopiidae (thresher sharks). The degree of endothermy varies from the billfish, which warm only their eyes and brain, to bluefin tuna and porbeagle sharks who maintain body temperatures elevated in excess of 20 °C above ambient water temperatures. See also gigantothermy. Endothermy, though metabolically costly, is thought to provide advantages such as increased contractile force of muscles, higher rates of central nervous system processing, and higher rates of digestion.

Fish are an important source of food in many cultures. Other water-dwelling animals such as mollusks, crustaceans, and shellfish are often called "fish" when used as food. For more details, see Fish food.

• Acanthemblemaria spinosa
• Altolamprologus
• Bala shark
• Brine shrimp
• Bubble nest
• Daphnia
• Deep sand bed
• Fish anatomy
• Fish diseases
• Gobiosoma multifasciatum
• Ichthyology terms
• List of freshwater aquarium fish species
• List of freshwater aquarium invertebrate species
• List of freshwater aquarium plant species
• List of marine aquarium fish species
• Live rock
• Marine aquarium
• Painted fish
• Protein skimmer
• Reef aquarium
• Saddleback clownfish
• Taxonomic classes
• Agnatha Hagfish
• Lamprey
  • Stephanoberyciformes
  • Stomiiformes
  • Synbranchiformes
  • Syngnathiformes
  • Tetraodontiformes
  • Zeiformes
  • Lobe-finned fish Lungfish
  • Prehistoric fish Acanthodii
  • Placoderms

Reptiles are tetrapods and amniotes, animals whose embryos are surrounded by an amniotic membrane. Today they are represented by four surviving :

• Crocodilia (crocodiles, caimans and alligators): 23 species
• Sphenodontia (tuataras from New Zealand): 2 species
• Squamata (lizards, snakes and amphisbaenids ("worm-lizards")): approximately 7,600 species
• Testudines (turtles): approximately 300 species
• Reptile | Agamidae | Archosaurs | Avicephalans | Fictional reptiles | Lepidosaurs | Marine reptiles | Parareptiles | Pet reptiles | Turtle |

Reptiles are found on every continent except for Antarctica, although their main distribution comprises the tropics and subtropics. Though all cellular metabolism produces some heat, most modern species of reptiles do not generate enough to maintain a constant body temperature and are thus referred to as "cold-blooded" or ectothermic (the Leatherback Sea Turtle is an exception). Instead, they rely on gathering and losing heat from the environment to regulate their internal temperature, e.g, by moving between sun and shade, or by preferential circulation — moving warmed blood into the body core, while pushing cool blood to the periphery. In their natural habitats, most species are adept at this, and can ususally maintain core body temperatures within a fairly narrow range, comparable to that of mammals and birds, the two surviving groups of "warm-blooded" animals. While this lack of adequate internal heating imposes costs relative to temperature regulation through behavior, it also provides a large benefit by allowing reptiles to survive on much less food than comparably-sized mammals and birds, who burn much of their food for warmth. While warm-blooded animals move faster in general, an attacking lizard, snake or crocodile moves very quickly.

Except for a few members of the Testudines, all reptiles are covered by scales.