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Fish tongues however do not resemble the muscular tongues of humans. The tongue of a fish is formed from a fold in the floor of the mouth.
In some species of bony fishes the tongue has teeth which help to hold prey items. The name of one genus of argentinid fish, Glossanodon, literally means 'tongue teeth'.
The tongue of the lamprey can be protruded from the mouth. It has horny teeth and is used to rasp flesh from its prey. Most fishes however cannot protrude their tongues.
The tongue and inside walls of the mouth of the Cottonmouth Trevally, Uraspis secunda, are brilliant white.
Several species of parasitic crustaceans (isopods of the genus Ceratothoa) are known as tongue biters. These parasites are sometimes seen on the floor of a fish's mouth 'replacing' the tongue. Current evidence suggests that the tongue is pierced at the base by the pereopods (walking legs) of the isopod, resulting in it eventually being physically detached, or atrophying and then dropping off.
Some species of tongue biters feed on the host's blood and many others feed on fish mucous. Tongue biters do not eat scraps of the fish's food. Most evidence indicates that these isopods do not kill the host.