What arctic sea whale has one long horn on its head?

The Arctic sea whale with one long horn on its head is the narwhal. That apparent horn is actually a long, spiraled tusk—usually an enlarged tooth extending from the upper jaw, most commonly in males.

Narwhals are medium-sized toothed whales that live in Arctic waters around Canada, Greenland, and Russia. Their scientific name, Monodon monoceros, is often translated as “one tooth, one horn.” The tusk can grow to roughly two to three metres long and contains millions of nerve endings, allowing it to respond to changes in the surrounding environment. Scientists believe it may help with sensing, display, and social behavior.

The narwhal is sometimes called the “unicorn of the sea,” but it is not a horn in the biological sense. Most males have a tusk, while some females grow one as well. Narwhals also have a second tooth that usually remains embedded. They feed on Arctic fish, squid, and shrimp, and depend heavily on sea ice, making them vulnerable to changes in the Arctic environment.