Pufferfish are notoriously dangerous to eat because they contain a deadly poison known as tetrodotoxin (TTX), which comes from their food. TTX accumulates in their livers, gonads, skin and intestines (all the parts that skilled Japanese fugu chefs learn to safely chop out).

TTX binds to a victim’s nerve cells, blocking signals and causing paralysis and often death by suffocation. Pufferfish don’t succumb to the poison because a genetic mutation stops the TTX from locking onto their nerves. This resistance has evolved repeatedly in various pufferfish species. Other animals, including snakes and toads, have evolved TTX resistance with the exact same genetic mutations.

Being immune to TTX gives pufferfish various advantages: predators learn to avoid them, allowing puffers to expand their diet and safely eat species contaminated with TTX. Male puffers have even evolved something of a fondness for it. Females smear TTX on their eggs, presumably to discourage predators from feasting on them, and males are attracted to the smell.