Fish are gill-bearing, aquatic, craniate creatures that lack digitized limbs. The live hagfish, lampreys, cartilaginous and bony fish, as well as numerous extinct related groups, are included in this concept. In the class Actinopterygii, ray-finned fish make up approximately 95% of all living fish species; 99% of these are teleosts.
The Cambrian period saw the emergence of the first soft-bodied chordates, which are the earliest animals that can be categorized as fish. They had notochords, which allowed them to be more nimble than their invertebrate counterparts despite the fact that they lacked a real spine. Throughout the Paleozoic era, fish would keep evolving and branching into a vast range of forms. Throughout the Paleozoic, many fish evolved exterior defenses against predators. The first fish with teeth emerged during the Silurian epoch, and many of them—including sharks—became ferocious marine predators rather than just arthropods’ prey.
Although some of the larger, more energetic swimmers, such as white sharks, and tuna, may maintain a greater core temperature, most fish are ectothermic (“cold-blooded”), enabling their body temperatures to vary as ambient temperatures change. Fish can interact audibly with one another, most frequently during feeding, aggressiveness, or courtship.
The majority of bodies of water are rich in fish. Although no species has yet been identified in the deepest 25% of the ocean, they can be found in nearly all aquatic environments, from the abyssal and even hadal depths of the deepest oceans (where they can be found as cusk-eels and snailfish) to the high mountain streams (where they can be found as char and gudgeon). Fish have more species diversity than any other group of vertebrates, with 34,300 documented species.
Fish are a valuable resource for people all throughout the world, particularly as food. Fish farmers raise fish in ponds or cages in the ocean or hunt them in wild fisheries by both commercial and subsistence fishermen (in aquaculture). Additionally, they are taken by anglers who fish for fun, are maintained as pets, raised by fishkeepers, and shown in public aquariums.
