From the Super Mario Wiki
Blue Electro-Koopas are a blue variant of Electro-Koopas, derived from Koopa Troopas, that appear in Super Mario Sunshine. They are one of the two types of Electro-Koopas, along with the red type, and are much smaller and more aggressive than the other type. Blue Electro-Koopas are found primarily in Pinna Park, but they also appear in the King Boo fight.
Blue Electro-Koopas throw their shells at Mario which can electrocute Mario and remove a portion of his health. The shell returns to the blue Electro-Koopa like a boomerang. To defeat them, Mario can either stomp them once they are exposed after throwing their shells, or spray them with water to make them vulnerable to their own shell. A defeated blue Electro-Koopa releases one coin when jumped on, but it drops four coins if it gets electrocuted by its own shell. If a blue Electro-Koopa throws its shell over a cliff, thus preventing it from returning, the shell disappears after a few moments, but the blue Electro-Koopa spontaneously grows a new one. Similarly, the shell will be destroyed if it lands in water, though it still takes some time before the Electro-Koopa replaces it.
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