Sweet shrimp

A sweet shrimp (アマエビ, Amaebi) is a type of deep sea creature found in. They are small and erratic, and it can be captured only by diving into it.

Donating to the Museum in New Leaf
The information board in the aquarium will list information about this deep-sea creature:

"Also called "northern shrimp," they have a life span of about 5 years, in which they are male for half. After that time, the shrimps change genders and become females, a rather unique feature of the species. The females are a popular sashimi dish, but they don't instantly have their characteristic sweetness. That sweetness actually comes out after they're left to sit for a day, making them a dish of patience."