The caudal fin, or tail fin, propels a fish through the water and enables it to swim. There are different tail fin shapes for different species of fish. The size and arrangement the fin determines how quickly and in what manner a fish swims. It can also be an indicator of a general type of fish and the environment in which it lives.
Ordinarily, caudal fins have an upper and lower lobe. In bony fish, the lobes are generally the same size and shape. This is called a homocercal tail fin. A homocercal tail ends at and connects to the fish’s spine. The swinging from side to side of the caudal fin allows the fish to propel itself through the water by what is termed carangiform locomotion.
A caudal peduncle is found at the base of the tail fin. It has various uses for different fish species, depending on their environment and natural defenses. Some fish have shortened tail fins roughly the shape of a triangle without separate lobes. Fish that inhabit the bottom of the body of water and camouflage themselves tend to have triangular caudal fins. Though not as fast as other fish, the design allows for a quick burst of speed to escape predators and hide.
Rounded tail fins are found on the fish that can be seen darting about in shallow waters. This shape of caudal fins is useful for fast maneuvering among rocks, grasses, and tight places. Unlike most other fish, the caudal fin of the eel is connected to all its side fins and wraps around the length of its long snakelike body. This allows for swift and strong swimming when hunting for food.
The caudal fins of the shark family are different than that of bony fishes. In some sharks, the vertebrae extend through the top lobe, making it longer than the lower lobe. This is referred to as a heterocercal tail. Other shark species have crescent shaped homocercal tails, sometimes called a lunate tail. The caudal peduncle of some sharks is flattened horizontally into ridge like keels, this helps to stabilize the shark during swimming.
There are some fish species that have no caudal fin. Instead, they use side fins or some other form of locomotion. A complete lack of a tail fin is rare among fish. Examples of this phenomenon are the ray family of fishes, with whip like tails, and seahorses, which swim upright.