The puffer fish has an alkaloid in its body called tetrodotoxin, which paralyzes the nerves and eventually the heart will stop beating and die as a result.
When the pufferfish is frightened, it will inhale a lot of air and water to make itself bulge up to four times its original size to intimidate intruders, and its exterior is covered with very hard spines, so that even enemies who want to eat him will lose their appetite.