The fins

Fish have several types of fins, each one playing a precise role. Their forms and names are often used to classify them into different families.

Xipho (Xiphophorus helleri), bred with overdeveloped fins.

Of the unpaired fins (i.e. consisting of a single fin), the most noteworthy are the dorsal and the anal fins. These serve to stabilize the fish when it is not going very fast or is coming to a halt, and they are tucked in when the fish swims more quickly. The caudal fin (incorrectly referred to as the tail) supplies propulsion, in conjunction with the rear part of the body. In some species, particularly the Characins and the catfish, there is a small extra fin between the dorsal and the caudal fins, known as the adipose fin. although this is not really used. The paired fins, attached symmetrically to each side of the body, are called pectoral and pelvic fins. They are used for stabilizing, stopping, slowing down, or changing direction: vertically, from the water surface to the bed, and vice versa, from side to side, from left to right, from right to left. Fins consist of a membrane stretched on spokes, and they can all be tucked in along the body, with the exception of the caudal fin. The adipose fin is merely a fold of skin, without any spokes. When the spokes are longer than the fins they are known as spiny fins, and they can represent a danger to the aquarist, as in the case of the scorpion fish, for example.