These piranhas are personal summons of Ruiga.
Abilities
Ruiga traps an enemy in an floating orb of water, then he summons a school of piranhas which attack the trapped target, slowly and painfully, considering they don't have sharp teeth like normal piranha.
Part I
Land of Vegetables Arc
Ruiga uses this on Kikunojō and later Naruto Uzumaki. Kikunojō died, but Naruto manages to escape death by the piranhas thanks to his comrades.