Home >> Science & Nature >> Animals & Science

Body Parts of a Fish: Fins

Fins are used by fish for swimming. They can also be used for gliding over the water, as in flying fish, or crawling. The fins of bony fishes are membranous and supported by stiff pointed spines or soft flexible rays. The fins of sharks and rays, whose skeletons are made of cartilage, are fleshy instead of membranous. The same is true of eel-like fish. The number and kind of fin rays or spines of a species can be used to identify it.

Pectoral and Pelvic Fins
- Some types of fins are paired, and there's one on each side of the body. The pectoral fins are located beside the operculum, or gill covering, or on the chest or behind the head. A fish such as the searobin has large winglike pectoral fins that have the first three rays detached from the rest of the fin. These rays serve as support when the fish is "walking" on the sea bottom. The pelvic or ventral fins can be anywhere on a fish between the throat and the abdomen.
Dorsal Fins
- There are three unpaired fins called the median fins. The dorsal fin is on the midline of the back and is usually divided into two or even three sections, which will be the first, second or the third dorsal fin. Some fish, like trout, have an additional small fin made of fat, called the adipose fin, on the midline of the back behind the dorsal fin. The first dorsal fin on a fish like the frogfish has been modified into a lure for prey. Some fish, like tuna, have finlets behind the dorsal and anal fins.
Anal and Caudal Fins
- The anal fin is on the underside of the fish, behind the anus. The caudal or tail fin is at the end of the caudal peduncle, the part of a fish's body between the posterior end of the anal fin base and the caudal fin base. Caudal fins can be rounded, truncated, emarginate, semilunar, forked, pointed, rhombic, or S-shaped. A heterocercal fin is a caudal fin where the upper lobe is much larger than the lower lobe, and contains the vertical column. This can be seen in a fish like the cow shark. A homocercal fin is a caudal fin where the lobes are about the same size, and neither one of them contains the vertical column. This can be seen in a goldfish.
Dangerous Fins
- In some species of fish the fin spines are sharp enough to puncture flesh. Sometimes the spines also inject venom. The dorsal, pelvic and anal fin spines in scorpionfish are venomous and the fish should be handled carefully. The veil like fins of the lionfish are beautiful, but the spines can inflict very painful wounds as well.

Animals & Science