Fish are gill-bearing aquatic animals that lack limbs with digits. The earliest organisms that can be classified as fish first appeared during the Cambrian period. Fish would continue to evolve through the Paleozoic era, diversifying into a wide variety of forms. Many fish of the Paleozoic developed external armour that protected them from predators. The first fish with jaws appeared in the Silurian period, after which many (such as sharks) became formidable marine predators. Most fish are ectothermic ("cold-blooded"), allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and tuna can hold a higher core temperature. Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams to the depths of the deepest oceans. With 33,600 described species, fish exhibit greater species diversity than any other group of vertebrates.
Fish are gill-bearing aquatic animals that lack limbs with digits. The earliest organisms that can be classified as fish first appeared during the Cambrian period. Fish would continue to evolve through the Paleozoic era, diversifying into a wide variety of forms. Many fish of the Paleozoic developed external armour that protected them from predators. The first fish with jaws appeared in the Silurian period, after which many (such as sharks) became formidable marine predators. Most fish are ectothermic ("cold-blooded"), allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and tuna can hold a higher core temperature. Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams to the depths of the deepest oceans. With 33,600 described species, fish exhibit greater species diversity than any other group of vertebrates.
