Oyster

An oyster (オイスター, Oisutā) is a type of deep sea creature introduced in. Despite their name, they are not related to pearl oysters.

Donating to the Museum in New Leaf
When donated to the museum, it is found in the primary, large tank in the first room of the fish exhibit. It is in the northeastern section of the tank and it is stationary. The information board says the following:

"Oysters are sometimes referred to as "the milk of the sea" because of their high nutritional value. Young oysters come from eggs, attach themselves to hard places in the ocean, and then grown. They change gender, allowing them to reproduce regardless of their neighbor's genders."