Fin of Fish: Fin Names, What Each Does, Tail Shapes

A fin is a thin flap of skin supported by bony spines or soft rays that a fish uses to move, steer and stay upright. Wikipedia explains the two kinds of support: spines are generally stiff and sharp and are never segmented; rays are generally soft, flexible and segmented, and may be branched. In bony fish, the rays (lepidotrichia) are usually made of bone and look like a stack of small discs.

Fins come in two groups (Wikipedia):

• Paired fins: the pectoral fins behind the gill cover, and the pelvic fins on the belly.
• Unpaired fins along the midline: the dorsal fin on the back, the anal fin underneath, and the caudal fin, or tail.

Some fish also have a small adipose fin on the back between the dorsal fin and the tail, and some have finlets.

• Dorsal fin (back): keeps the fish from rolling and helps it make sudden turns and stops (Wikipedia). A fish may have one dorsal fin, or two that are joined or separate (Idaho State University, Trout in the Classroom guide).
• Anal fin (underside, in front of the tail): stabilises the fish like the keel of a boat (Idaho State University); most fish use it to stay steady while swimming (Wikipedia). Deep-bodied fish such as bluegill have longer dorsal and anal fins for stability.
• Caudal fin (tail): provides the push that drives most fish forward and also works as a rudder (Wikipedia; Idaho State University).
• Pectoral fins (sides, behind the gills): help steer and control depth; in bluegill and crappie they sit high near the gill cover and work almost like wrists for precise moves among plants (Idaho State University). They correspond to the forelimbs of four-legged animals (Wikipedia).
• Pelvic fins (belly): help the fish go up or down, turn sharply and stop quickly, and keep it from rolling; some fish rest on them (Wikipedia; Idaho State University).
• Adipose fin (small fleshy fin behind the dorsal): its job is still being studied; it may reduce drag, and research suggests it may help the fish sense touch, sound and pressure changes (Wikipedia; Idaho State University).

Yes. The tail is the caudal fin, from the Latin cauda, meaning tail (Wikipedia). The narrow part of the body just in front of it is the caudal peduncle.

Tail shape tells you how a fish lives. Idaho State University's guide sorts tails into forked, heart-shaped, square and rounded: fish that cruise all day looking for prey have forked tails, which create less drag, while fish with rounded tails are the least speedy. Wikipedia adds truncate, emarginate (slightly notched) and lunate (crescent) shapes.

The tail skeleton differs too. Sharks usually have a heterocercal tail, with the backbone running into the larger upper lobe; most modern bony fish have a homocercal tail that looks symmetrical from outside (Wikipedia).

Wikipedia gives several examples of fins with other jobs:

• frogfish walk along the sea floor on their pectoral and pelvic fins;
• flying fish glide above the water;
• stonefish have venomous dorsal spines for defence;
• anglerfish have a dorsal spine modified into a fishing lure.

Gobies are another case: their pelvic fins are fused into a sucker disc that grips rocks (Wikipedia), which is also the quickest way to tell a goby from a sculpin, whose pelvic fins are separate (USGS). Venomous spines are the reason to handle unknown spiny fish with a tool, not bare hands.

Fins are one of the most reliable identification clues, because colour can change (the foxface rabbitfish turns a dull mottled pattern at night, Wikipedia) while fin structure does not. What to check:

• Is there an adipose fin? A small fleshy fin between the dorsal fin and the tail narrows a fish down to groups such as the salmon and trout family, characins and catfish (Wikipedia; Idaho State University). If that fin is missing on a trout or salmon, it may have been clipped: the adipose fin is often removed to mark hatchery-raised fish (Wikipedia).
• One dorsal fin or two? A fish may have a single dorsal fin, or two that are joined or separate (Idaho State University); note which, and where the stiff spines give way to soft rays.
• Spines versus soft rays. FishBase lists fin counts as spines plus soft rays. For example, it gives the giant trevally 9 dorsal spines and 18 to 21 dorsal soft rays.
• Tail shape: forked, square, rounded or lunate (see above).
• Pelvic fins: fused into a sucker (gobies), separate (most fish) or missing altogether, as in pipefish (Wikipedia).

For a photo, lay the fish flat and gently spread the dorsal fin so every spine can be counted, then photograph the tail and the belly fins. Our guide to key features for identifying fish covers the other marks to check.

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