Giant Clam

This is the personal summon of the Second Mizukage.

Shinobi World War Arc
During the Fourth Shinobi World War, after the Second Mizukage is resurrected by Kabuto Yakushi to fight for Akatsuki, it is summoned to the battlefield to fight the shinobi of the Fourth Division.

Trivia

 * Although simply called 'giant clam', the kanji is actually the name for a mythical giant clam and is usually pronounced as 'shin' in Japanese ('shèn' or 'chèn' in Chinese). This shin is actually a kind of aquatic, shape-shifting dragon in the form a giant clam. It is said to be able to cause mirages and Fata Morganas. In fact, its name has become almost synonymous with 'mirage': in Japanese, the word for 'mirage' is.
 * The mirage-theme of the giant clam is a reference to how the Second Mizukage uses genjutsu.