Natural defenses of the pufferfish
Its backup defense mechanism, used if successfully pursued, is to fill its extremely elastic stomach with water (or air when outside the water) until it is much larger and almost spherical in shape. Even if they are not visible when the puffer is not inflated, all puffers have pointed spines, so a hungry predator may suddenly find itself facing an unpalatable, pointy ball rather than a slow, tasty fish. Predators which do not heed this warning (or which are "lucky" enough to catch the puffer suddenly, before or during inflation) may die from choking, and predators that do manage to swallow the puffer may find their stomachs full of tetrodotoxin, making puffers an unpleasant, possibly lethal, choice of prey. This neurotoxin is found primarily in the ovaries and liver, although smaller amounts exist in the intestines and skin, as well as trace amounts in muscle. It does not always have a lethal effect on large predators, such as sharks, but it can kill humans.