Fins are locomotory organs of fishes that propel, steer and turn the body in water which is a viscous medium when compared to terrestrial situations. Fishes possess two pairs of paired fins, namely, pectoral and pelvic which are used for steering, braking, balancing and pitching. Caudal fin propels the body forward and also neutralizes drag against water. Dorsal and anal fins are chiefly antirolling devices and hence are well developed in active fishes.
TYPES OF CAUDAL FINS
Depending on the habits and habitats caudal fins are variously modified in fishes.
Found in primitive vertebrates like cyclostomes, this type of fin has only one lobe inside which vertebral column extends up to the tip. Lateral undulation of the whole body propels body forward. The fin may be continuous with the dorsal fin.
Found in bottom dwelling Holocephali, the fin is similar to protocercal type in structure but the vertebral column extends beyond the apex of the fin in a whip like shape. The fin is suitable for swimming near the bottom of the sea.
The vertebral column stops at the base of fin and the fin itself has a single large uniform lobe that lashes side to side. Ex. Lungfish and Latimeria.
The fin is divided into two uniform sized lobes to increase its surface area. Vertebral column extends up to the base of the fin and bends slightly upwards. Majority of bony fishes possess this type of caudal fin.
A characteristic feature of elasmobranchs, heterocercal tail fin has large dorsal lobe and a smaller ventral lobe. Vertebral column extends into the upper lobe so that it can be moved sideways with force. This type of fin produces a lift force on the tail and keeps the head down during swimming, a feature quite suitable for bottom living fishes.
In flying fishes lower lobe of the caudal fin is highly enlarged, so that it produces a downward force on the tail. While swimming the head is naturally lifted up and the tail pulled down by hypocercal tail fin, facilitating the fish to swim upward in water and then jump out of water to glide in air to considerable distance to escape predators.
TYPES OF PAIRED FINS
Also called archipterygium, they are found in lung fishes. They have a series of axial bones which form the axis of the fin, to which are attached small radial bones on either side. Radial bones provide a base for the attachment of tissues and muscles of the fin. These fins are very weak in lung fishes except in the Australian lung fish in which they are also used for walking at the bottom of the river.
Found in elasmobranchs, this fin has 1-5 basal cartilages that form a strong base for the attachment of muscles. Many radial cartilages are attached on the distal side of the basals. Fin itself is a large mass of the muscles, tissues and skin.
Ray fins possess bony spine-like fin rays attached to small rounded radials which in turn are attached to coraco-scapula bone of the pectoral girdle. Skin is stretched between fin rays due to which the fin can be folded or stretched at will.
They are found in Latimeria (Crossopterygii) and have a lobed stalk and a broad terminal fin lobe. The three basal bones exhibit homology with the tetrapod humerus, radius and ulna. Radials can be compared with metacarpals and phalanges. It is believed that lobe fins evolved into tetrapod limbs during Devonian period when amphibians evolved.