What is the fin on the backs of fish, some whales and dolphins called?

The fin on the backs of fish, some whales and dolphins is called a dorsal fin. “Dorsal” means related to the back, so the name describes its position on the animal’s body.

In most fish, the dorsal fin helps stabilize the body and prevents rolling as the animal swims. It can also assist with turning, defense, courtship displays, or propulsion in specialized species. Sharks often have two dorsal fins, while tuna commonly have two that can be folded into grooves as they swim.

Dorsal fins evolved independently in several aquatic animal groups, including fish and cetaceans such as whales and dolphins. Their similar streamlined shapes reflect adaptation to life in water rather than close evolutionary relationship. In cetaceans, the fin is made of connective tissue rather than the bony fin rays found in many fish.

A common mix-up is confusing the dorsal fin with the caudal fin, which is the tail fin. The pectoral fins are the paired side appendages used mainly for steering. Not every whale or dolphin has a dorsal fin: belugas and right whale dolphins are notable exceptions.