Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of living fish species are ray-finned fish, belonging to the class Actinopterygii, with around 99% of those being teleosts. The earliest organisms that can be classified as fish were soft-bodied chordates that first appeared during the Cambrian period. Although they lacked a true spine, they possessed notochords which allowed them to be more agile than their invertebrate counterparts. Fish would continue to evolve through the Paleozoic era, diversifying into a wide variety of forms. Many fish of the Paleozoic developed external armor that protected them from predators. The first fish with jaws appeared in the Silurian period, after which many became formidable marine predators rather than just the prey of arthropods.

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Fish sensory organ key to improving navigational skills of underwater robots

New Fish Species Has Mysterious Horn-Like Structure on the Back of Its Head

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Ghost knifefish make electric 'chirps' to spot where other fish are

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Invasive rats transform reef fish behavior

Reef fish are more placid around islands infested with invasive rats

Over 330 fish species -- up to 35 new to science -- found in Bolivian national park

Reef fish must relearn the 'rules of engagement' after coral bleaching

Fish that have lost food due to mass coral bleaching are getting into more unnecessary fights, causing them to expend precious energy and potentially threatening their survival, according to new research

Fish see in the dark thanks to extra layers of rod cells in their eyes

Insights get the right fish on the right plate

New study sheds light on boric acid transport and excretion in marine fish

Fish larvae find their way using external cues