Dorsal fin

A dorsal fin is a fin on the back of most marine and freshwater vertebrates. These fins help fish and other water animals stay steady and move easily through the water. Dorsal fins have appeared in many different animals over time, even though they look different in each species.

You can see dorsal fins in most fish, as well as in large mammals like whales. Some animals had them long ago, such as ancient marine reptiles like ichthyosaurs. While many have just one dorsal fin, a few have two or even three.

Scientists, called biologists, often study the special shapes and marks on whale dorsal fins to tell different whales apart. Inside the fins, special bones or cartilages called pterygiophores help support the fin.

Functions

The main purpose of a dorsal fin is to help the animal stay balanced and make quick turns. Some fish have special uses for their dorsal fins. For example, the sunfish uses its dorsal fin and the anal fin to move forward. In anglerfish, the front part of the dorsal fin looks like a fishing pole with a lure to catch prey.

Some fish use their dorsal fins to stay safe from predators. They may have sharp spines or even venom, like many catfish, the spiny dogfish, and the Port Jackson shark.

Dorsal fins come in different numbers and shapes. Most fish, like the large goldfish, have one dorsal fin. Sharks, the yellowfin tuna, and haddocks usually have two. Some haddocks have three.

Billfish have very noticeable dorsal fins. When they swim, they can pull these fins into a groove in their body to move faster. The shape and size of the dorsal fin can help tell different kinds of billfish apart. The great white shark’s dorsal fin has special fibers that help it stay steady when swimming fast.

Some fish have dorsal fins with special jobs. The Gymnarchus niloticus uses its long dorsal fin to move while keeping its back straight. Many catfish can lock their dorsal fin spines up to scare away predators. The large, fold-up dorsal fin of the Indo-Pacific sailfish may help control the fish’s temperature.

Dorsal fin shapes can differ even within the same kind of animal. Male and female orcas have dorsal fins that look different, showing their sex. Long ago, ichthyosaurs also had many different shaped dorsal fins.

Structure

A dorsal fin is a special fin on the back of many water animals. It helps them stay steady while they swim. In fish, this fin grows from skin folds and is supported by tiny bones and muscles that help the fish move through the water. The fin has two main parts: soft rays and a strong base with muscles.