What structure do beluga whales lack that most other toothed whales have?

Beluga whales lack a dorsal fin.

Instead, belugas have a low dorsal ridge along their backs. This streamlined profile is useful in an environment where they regularly swim beneath sea ice: a tall fin could be vulnerable when the whale surfaces through narrow openings or moves under frozen cover. Belugas also have flexible necks, allowing them to turn their heads more readily than many other whales.

A missing dorsal fin does not mean a beluga lacks other features typical of toothed whales. It has a blowhole, a tail fluke, and numerous teeth used to seize fish and invertebrates. The narwhal, its closest living relative, also lacks a conventional dorsal fin. Belugas are known as “white whales,” but calves are born gray or brown and gradually become pale as they mature.