Giant Clam

This is the personal summon of the Second Mizukage.

Appearance
Aside from its massive size, it resembles an ordinary clam. Several prominent dark lines encircle both the upper and lower portions of its shell, while the Kirigakure symbol is inscribed upon its top.

Abilities
The creature is capable of producing a deceptive mirage that completely renders observers unable to accurately locate the position of itself or its summoner, after firstly opening its shell and exuding a mist from its siphons throughout the duration of the technique.

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 in a confrontation against the shinobi of the Fourth Division. As the clam opens its shell, it envelops the area in a mirage that renders all the attacks the shinobi dealt ineffective, as they remain unable to locate the actual position of either the creature or the Mizukage. Despite the Mizukage's advice to attack the clam standing directly behind him, their attempts continue to fail, resulting in utter confusion amongst the shinobi. After Gaara and Ōnoki arrive, the real clam is located by Gaara's Sand Sensing and Ōnoki uses an enhanced Earth Release: Fist Rock Technique to destroy it. Due to this, the mirage disappears.

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.
 * This and other references to mirages relating to the giant clam are a further allusion to the Second Mizukage's use of genjutsu.