Sea butterfly

The sea butterfly (クリオネ, Kurione) is a species of mollusk that live in the sea. They can be caught in the during winter and are the only winter fish that cast such a small shadow in the sea, and so can be identified with ease.

Donating to the Museum
The sea butterfly can be donated to the Museum by talking to Blathers. Below is what he says during the donation:

In Wild World
"The sea butterfly is in the same family as snails, as you may or may not know. You might think it's strange to have a snail without a shell, eh wot? Indeed, well, slugs are also snails without shells! I must admit, being lumped in with slugs probably angers the sea butterfly.

The sea butterfly is placed in the large ocean tank in the aquatic display in the back room.

In City Folk
It occupies a large glass tube in the lower-right corner of the fish exhibition, with other small oceanic fish.

Encyclopedia Information
Once the player has caught a particular fish or bug, they can find information about it in the bug or fish menus. Once the sea butterfly has been caught, the following information can be found in the fish menu:

Further information
Sea butterflies can be found in the cold waters of the Arctic Ocean, Antarctic Ocean and in the North Atlantic; mirrored, in-game, by their appearance during the winter months. They can grow up to 5 centimeters in length.