Fish are a broad assemblage of aquatic vertebrates that breathe mainly through gills and live in fresh or salt water. The common English term includes animals with true bones as well as those whose skeleton is mainly cartilage. Fish respire by extracting oxygen dissolved in water, typically via gill structures. Historically the word "fish" was treated as a single biological class, but modern biology recognizes several distinct lineages and multiple classes that are commonly called fish.
Major groups and diversity
Modern usage groups fish into several principal kinds: jawless fish, which include hagfish and lampreys; various extinct armoured forms known from fossils; living cartilaginous fish such as sharks and rays; the very diverse ray-finned fish that make up the majority of species; and the lobe-finned fish, a small group that includes the ancestors of all land vertebrates. There are more described species of fish than of terrestrial tetrapods—well over thirty thousand—ranging from tiny gobies to gigantic sharks.
Characteristics and anatomy
- Respiration: Most fish use gills to take up oxygen; some species have adaptations for air-breathing in low-oxygen water.
- Fins and movement: Fish typically have paired fins for steering and unpaired fins for stability; body shapes and fin arrangements vary with habitat and lifestyle.
- External coverings: Many fish are covered in scales or plates; others are smooth-skinned.
- Buoyancy and senses: Ray-finned fish commonly possess a swim bladder to control buoyancy, while sharks rely on oil in their liver. A lateral line detects water movements, and several species have electroreception.
- Temperature regulation: Most fish are ectothermic (often called cold-blooded) and their body temperature varies with the environment.
Habitats, life histories and noteworthy examples
Fish inhabit a wide range of environments: some live in fresh water in lakes and rivers, while others occupy coastal and open-ocean zones in salt water. Sizes range from species under a centimeter to the enormous whale shark, reported to reach nearly 15 meters and weighing up to about 15 tons. Some fish show remarkable adaptations: lungfish can breathe air with rudimentary lungs and survive drying conditions by burrowing and entering dormancy; certain species are known to aestivate until water returns.
Evolution and classification challenges
From a cladistic perspective the everyday term "fish" is paraphyletic: it groups animals that do not all share a single common ancestor to the exclusion of other vertebrates. Discussions of cladistics emphasize that some animals called fish are more closely related to tetrapods (four-limbed vertebrates) than to other fish. Notably, lobe-finned fish gave rise to the earliest four-limbed land vertebrates, making them the ancestors of amphibians, reptiles, birds and mammals.
Human uses, ecological roles and conservation
Fish are central to many human societies as sources of protein (commercial and subsistence fisheries), livelihoods, cultural practices and recreation. They also play essential ecological roles as predators, prey and participants in nutrient cycles. At the same time many fish populations face threats from overfishing, habitat destruction, pollution, dams on rivers, invasive species and climate change. Conservation measures include catch limits, protected areas, habitat restoration and aquaculture management. Understanding the biology and diversity of fish is important for sustainable use and for preserving aquatic ecosystems.
For further reading, consult general references on vertebrate biology and fisheries science; use the linked keywords in this article as starting points for more detailed topics.
