Plumping up isn't the only protective option for a puffer fish. It also can defend itself via its onboard tetrodotoxin -- one of the most toxic poisons known. But unless we're actually eating fugu, as Japanese gourmands call it, it's the puffing that typically comes to mind. It's right there in the name.
Until recently, scientists thought that puffer fish accomplished their ballooning by "holding their breath" -- that is, by shutting down their gills and pulling in oxygen from the surrounding water using capillaries in their skin [sources: Diep; Zielinski]. It's an image in keeping with the fish's appearance -- the pursed lips, the wide eyes, the engorged Dizzy Gillespie "cheeks" -- but it also turns out to be a lot of hot air.
According to a study published in the December 2014 edition of Biology Letters, the puffery has more to do with gulping than gasping. It turns out that a puffer fish actually bloats itself by trapping large gulps of seawater in its stomach, then clamping the organ off at either end. Far from holding its breath, it continues to respire through its gills all the while, which is just as well, because the process costs the fish dearly in terms of oxygen consumption and is therefore quite tiring [sources: Diep; McGee and Clark; Zielinski].