Giant Clam

This is the personal summon of the Second Mizukage.

Appearance
The Giant Clam resembles any normal clam with the exception of have several dark lines, outlined on the top of its shell. It also has the Kirigakure symbol inscribed on top of it as well.

Abilities
When the clam opens its shell, it releases a mirage that completely renders people unable to tell where it or its summoner is.

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. As the clam opens its shell, it envelops the area in a mirage rendering all the attacks the shinobi dealt ineffective. As the Mizukage tells them to attack the clam standing right behind him, after several attempts all of which failed, the shinobi end up in utter confusion.

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.