Biologists think pufferfish, also known as blowfish, developed their famous “inflatability” because their slow, somewhat clumsy swimming style makes them vulnerable to predators. In lieu of escape, pufferfish use their highly elastic stomachs and the ability to quickly ingest huge amounts of water (and even air when necessary) to turn themselves into a virtually inedible ball several times their normal size. Some species also have spines on their skin to make them even less palatable.
A predator that manages to snag a puffer before it inflates won’t feel lucky for long. Almost all pufferfish contain tetrodotoxin, a substance that makes them foul tasting and often lethal to fish. To humans, tetrodotoxin is deadly, up to 1,200 times more poisonous than cyanide.