Pufferfish will “puff up” as a defense mechanism if they are threatened. … Even though they sometimes do it just to stretch, puffing up often can be stressful for a puffer. So, it’s best if they do it only when they need to. This behavior isn’t a puffer’s only means of defense.
Do puffer fish fill up with water or air?
Pufferfish can inflate into a ball shape to evade predators. Also known as blowfish, these clumsy swimmers fill their elastic stomachs with huge amounts of water (and sometimes air) and blow themselves up to several times their normal size. Some pufferfish species also have spines on their skin to ward off predators.