Which Arctic marine mammal is known for having no dorsal fin?

The beluga whale is an Arctic marine mammal with no dorsal fin.

Instead of a projecting fin, the beluga has a low dorsal ridge. This streamlined shape helps it move beneath sea ice, where a tall fin could make travel and surfacing more difficult. Belugas can also bend their necks more than most whales, helping them maneuver in shallow or ice-filled water.

Belugas are easily recognized by their white adult color, rounded forehead, and extensive vocal repertoire. They are sometimes called “sea canaries” because of their whistles, clicks, and other sounds. Calves are born gray or brown and gradually become white as they mature.

The narwhal is the beluga's closest living relative, but narwhals also lack a conventional dorsal fin and have a tusk. That means the finless feature alone cannot identify every monodontid. Belugas are generally smaller than narwhals and often enter estuaries and river mouths during summer.