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Learn about the definition, classification, and history of fish, the aquatic vertebrates with fins and gills. Explore the fossil record, phylogeny, and diversity of fish groups, from jawless to bony fish, and their relationship with tetrapods.
- Overview
- Structural diversity
A fish is any of approximately 34,000 species of vertebrate animals. The term fish is applied to a variety of vertebrates of several evolutionary lines. It describes a life-form rather than a taxonomic group.
How do fish sleep?
When a fish sleeps, it exists in a seemingly listless state in which the fish maintains its balance but moves slowly. If attacked or disturbed, most fish can dart away. A few kinds of fish lie on the bottom to sleep. Most fish do not have eyelids, so they cannot close their eyes to sleep.
How do fish hear?
The organs of hearing in fish are entirely internal, located within the skull, on each side of the brain, and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body to be transmitted to the hearing organs.
fish, any of approximately 34,000 species of vertebrate animals (phylum Chordata) found in the fresh and salt waters of the world. Living species range from the primitive jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes. Most fish species are cold-blooded; however, one species, the opah (Lampris guttatus), is warm-blooded.
Fishes have been in existence for more than 450 million years, during which time they have evolved repeatedly to fit into almost every conceivable type of aquatic habitat. In a sense, land vertebrates are simply highly modified fishes: when fishes colonized the land habitat, they became tetrapod (four-legged) land vertebrates. The popular conception of a fish as a slippery, streamlined aquatic animal that possesses fins and breathes by gills applies to many fishes, but far more fishes deviate from that conception than conform to it. For example, the body is elongate in many forms and greatly shortened in others; the body is flattened in some (principally in bottom-dwelling fishes) and laterally compressed in many others; the fins may be elaborately extended, forming intricate shapes, or they may be reduced or even lost; and the positions of the mouth, eyes, nostrils, and gill openings vary widely. Air breathers have appeared in several evolutionary lines.
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Many fishes are cryptically coloured and shaped, closely matching their respective environments; others are among the most brilliantly coloured of all organisms, with a wide range of hues, often of striking intensity, on a single individual. The brilliance of pigments may be enhanced by the surface structure of the fish, so that it almost seems to glow. A number of unrelated fishes have actual light-producing organs. Many fishes are able to alter their coloration—some for the purpose of camouflage, others for the enhancement of behavioral signals.
Explore the diversity and evolution of fish, the vertebrates that live in water and have a backbone. Learn about their habitats, senses, adaptations, and threats through photos, stories, and videos.
Jan 3, 2023 · Learn about the diversity, characteristics, evolution, and exceptions of fish, the aquatic vertebrates with gills, fins, and scales. Find out how fish are classified, what they eat, and how they reproduce.
Jul 28, 2022 · Learn everything about fish, from their definition, ecology, types, and life cycle. Explore the diversity and adaptations of fish, from jawless to bony, from sharks to salmon.
Fish (plural: fish or fishes) are a group of animals which live in water and respire (get oxygen) from their gills. As a group, they are much older than other vertebrates. The first fish developed about 500 million years ago. [4] Fish used to be a class of vertebrates. Now the term covers five classes of animals that live in the water: Jawless fish
Learn about the diversity, evolution, and adaptations of fish, the predominantly cold-blooded vertebrates that live in water. Find out about the external features, respiration, reproduction, and classification of fish, as well as some examples of shark species.
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