First, narwhals aren't fish, they're mammals-‑a rare type of whale.
They have a famous spiraled tusk. The tusk, which is commonly misconceived for a horn, is actually an elongated tooth. Only male narwhals grow tusks, which can be over six to almost ten feet long--more than half their body length.
What Is The Function Of The Tusk?
There is patchy evidence for several different hypotheses. Since only males grow tusks, it could have evolved in response to sexual selection. Like the tails of male peacocks, the tusk could be used to attract and impress females and intimidate rival males.
Males may use their tusks in fights with other males to establish social hierarchies.
Tusks could also be used for breaking through thin layers of ice in their frigid ocean habitat.
Researchers from Harvard's School of Dental Medicine think the giant tooth might also act as a sensory organ. With an electron microscope they discovered that the surface of a narwhal tusk is covered with millions of tiny nerve endings. These nerves connect to the narwhal's brain through a network of tiny tubes in the tooth.
The scientists believe that the narwhal use the sensitive tusks to detect changes in temperature, salt content, and flow patterns in the environment around them.
But since only males grow tusks, it's likely that the tusks did not evolve primarily as a sensory organ. More research is needed to really understand why narwhals have tusks and how they use them.