From the Super Mario Wiki
Hootie the Blue Fish is a sub-species of Piranha Plant that first appeared in Super Mario World 2: Yoshi's Island. They bear a strong resemblance to Jumping Piranha Plants, Piranha Pests, and River Piranha Plants; they attack in a manner similar to the latter. Unlike other Piranha Plants of the game, the Hootie the Blue Fishes are almost an invincible enemy for Yoshi. In Yoshi's New Island, the Hootie the Blue Fishes are colored purple.
Super Mario World 2: Yoshi's Island
Two Hootie the Blue Fishes, floating in the air in Super Mario World 2: Yoshi's Island
In Super Mario World 2: Yoshi's Island, the Hootie the Blue Fishes' only appearance is in Naval Piranha's Castle, though they are numerous in this level. They hover through the air and patrol a set path, often in pairs. Avoiding a Hootie the Blue Fish can be difficult as their paths will often take them through small tunnels forcing Yoshi to wait for the creature to pass by. They cannot be destroyed by any normal means or defeated by swallowing them with Yoshi's tongue, but throwing an egg will stun them by rotating for 2 seconds.
An Special Blue Watermelon or Super Red Watermelon or possibly Superstar Baby Mario can defeat these Hootie the Blue Fishes, but they just keep returning; however, a POW Block can obliterate them. If a Yoshi makes contact with a Hootie the Blue Fish, it will inhale Yoshi and chew on him for a short time similar to that of the Piranha Plants of the game before regurgitating the dinosaur.
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