Which whale species is known for its long, spiral tusk?

The whale species known for its long, spiral tusk is the narwhal.

The narwhal, Monodon monoceros, is an Arctic toothed whale famous for the tusk that usually projects from the left side of a male's upper jaw. The tusk is not a horn: it is an elongated canine tooth with a spiral structure and millions of nerve endings. It can grow throughout the animal's life and may reach roughly 3 metres, although size varies.

Most males develop a prominent tusk, while females generally do not. A small percentage of females grow one, and some males develop two when the right canine also emerges. Researchers regard the tusk primarily as a sexually selected trait associated with display and social status, although observations suggest it may also help with sensing the environment and interacting with prey.

Narwhals live mainly in Arctic waters and are closely related to belugas. They are not simply miniature unicorns of the sea, though their tusks inspired that legend and were historically sold in Europe as supposed unicorn horns. Bowhead, sperm, and beluga whales do not possess this characteristic spiral tusk.