The dorsal fin is the fin on the top part of an aquatic animal. Most fish (including sharks) posess a dorsal fin, and dolphins and killer whales do as well, but most other species of whales do not.
The purpose of the dorsal fin is to increase the animal's stability in the water, and prevent it from rolling over. The animal may have one or more dorsal fins, depending on its species. If it has multiple fins, they are referred to by number (i.e. the first dorsal fin, the second dorsal fin), starting from the fin closest to the head and moving back.
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