Chambered nautilus

The Chambered Nautilus (オウムガイ, Ōmugai) is a type of sea creature introduced in.

In
Upon donating a Chambered Nautilus to the museum, it can be found in the small tank on the left hand side in the first room of the sea exhibit. The exhibit has this to say about the Chambered Nautilus:

"These creatures are related to both octopi[sic] and squids and originated about 60 million years ago. They have air and body fluid in their shells and move by adjusting the amount of body fluid inside. This allows them to either float or sink depending on the situation they find themselves in. They have nearly 90 tentacles that stick out from under their shells to grab on to rocks or prey."

In
When donating to the museum, Blathers will say the following:

"The chambered nautilus is perhaps best known for its gorgeous shell. Not only does this shell's interior shine with a pearly luster, it features a near-perfect natural spiral. But those aren't the chambered nautilus's only bragging rights. This cephalopod can have up to 90 tentacles, it's said! These arms come coated with a sticky substance that helps the nautilus capture its prey... Which is far better than using then for overly long hugs, I say."

Real-world information
The chambered nautilus is the best-known species of nautilus. The shell, when cut away, reveals a lining of lustrous nacre and displays a nearly perfect equiangular spiral, although it is not a golden spiral. The shell exhibits countershading, being light on the bottom and dark on top. This is to help avoid predators, because when seen from above, it blends in with the darkness of the sea, and when seen from below, it blends in with the light coming from above.

Nautilidae as a family, both extant and extinct, are characterized by involute or more or less convolute shells that are generally smooth, with compressed or depressed whorl sections, straight to sinuous sutures, and a tubular, generally central siphuncle. Having survived relatively unchanged for hundreds of millions of years, nautiluses represent the only living members of the subclass nautiloidea.

Although once thought to be a living fossil, the chambered nautilus is now considered taxonomically very different from ancient ammonites, and the recent fossil record surrounding the species shows more genetic diversity among nautiluses now than has been found since the extinction of the dinosaurs. Indeed, the taxon of the chambered nautilus, Nautilus pompilius is actually a grouping of tens of different species of nautilus under one name.