What is the function of fish tongue?

Answer 1

The name of the bone forming the "tongue of a fish" is the basihyal. A basihyal is a small, thick, relatively immovable piece of either bone or cartilage that is found on the floor of the mouth of fishes. It is in the front of the basibranchial corpula, a tube made of separate sections (the basibranchial bones) that contains the ventral aorta. The ventral aorta leads blood from the heart of the fish to the base of the gill arches.
Some fish have teeth on the basihyal that files down prey coming down the fish's throat. Rainbow trout have teeth on the basihyal that may be used for this type of food processing. Note that the basihyal is immobile. So this filing isn't the same as chewing or licking. The "cookie cutter shark" uses the basihyal, along with sharp teeth and suction, to rip "flesh cookies" out of their prey. The function of the basihyal in these cases seems to be analogous to part of the jaw.
The basihyal varies in importance with fish. The "Amilia calva" is a fish that has no separate basihyal. Most sharks don't seem to use the basihyal for anything, with the exception of the cookie cutter shark. For most fish, the use of the basihyal is uncertain.
In fish, the taste buds are usually contained in either the barbels or the lateral line. Sharks have taste buds all over their skin. However, the basihyal has not been shown to be covered with taste buds. A mammal tongue is covered with taste buds. So the basihyal and the mammal tongue don't seem to be homologous. If you consider the mammal tongue to be the "true" tongue, then the basihyal is a false tongue.
The basihyal is not considered a true tongue by most scientists. The tongue of most vertebrates (not fish) is a muscle, not a bone. The tongue of most vertebrates is used to both move food around and to taste food. The basibranchial is a bone, and has no muscles to move it around. Therefore, its resemblance to a mammal tongue is coincidental.
The entire skin of sharks is covered with taste buds, including the skin that covers the basihyal. Therefore, the basihyal does not seem to be a special "tasting" organ. However, in mammals only the tongue has taste buds. In both mammals and fish, there are specialized olfactory cells that only exist in the nasal cavities. The basihyal is not involved in smell. Therefore, the basihyal does not seem to be specialized for chemical sensing.
I conjecture that the most important function of the basihyal is to protect the heart and ventral aorta from impact with the food of the fish. The heart of a fish and the aorta are very close to the mouth of the fish. If there was nothing blocking contact with the basibranchial bones, then a frontal approach by a large mass of food could crush the aorta or the heart. However, it is unclear in most fish how important the "basihyal" is for survival.
The evolution of the basihyal may be a case where an almost vestigial organ developed new functions. The basihyal is homologous to the basibranchial bone of the second gill arch. As a basibranchial, it provided blood to the other bones of the second gill arch. Most of the bones from the first two gill arches were incorporated into the jaws of modern fish. However, the basihyal may have been a left over bone. It lost its ability to provide blood to the other bones. Although not completely vestigial, it was left with only a protective function. Later adaptations may have given the basihyal different uses.

Answer 2

A lions tongue is used for grooming and eating. When eating the tongue holds the meat and the barbs on the lions tongue is enough to take the skin off your hand if it licked you.

Answer 3

In short, to provide friction for food handling and contains taste buds.

Answer 4

The frog uses it to snare food and bring the food to its mouth.

Answer 5

To catch food, to allow a passage of water over the gills, to act as a nursery (in mouth breeding cyclids).