Writing for her blog Science Sushi at Discover Magazine, Christie Wilcox explains that pufferfish poison is no joke:
Pufferfishes in the genus Takifugu are known for their poisonous nature. Any predator that messes with these toxic fish learns the hard way that their tissues are loaded with tetrodotoxin, one of the most deadly poisons on the planet. It can kill a wide diversity of species, from fish to mammals, because it’s a potent paralytic that shuts down ion channels vital for nerve functioning. Humans are not immune: tetrodotoxin killed 179 people and poisoned another 467 in Japan alone from 1974 to 1983, where the flesh of these fish, fugu, is considered a delicacy perhaps because of the danger involved.
In fact, the toxin is 120,000 times deadlier than cocaine -- deadlier even than cyanide. Just a few milligrams can kill a human adult. That's why preparing the fish for human consumption is such a dangerous game: Patrons often want to taste just the tiniest tingle in their mouths from the poison to remind them how adventurous their meal is, but it only takes a small miscalculation to wipe your customers out before they can get to dessert.
By all appearances, however, larval pufferfish are just as helpless as any other baby fish. Quickly abandoned by their parents, they're not able to puff themselves up to visually intimidate predators quite yet, and they haven't accumulated enough toxins in their body to deter anyone who takes a bite.
In a paper published this month in Toxicon, researchers showed that larval pufferfish have more tetrodotoxin than they should. But it's not coming from the inside out; it's spread all over the surface of their skin. It turns out that extra tetrodotoxin is stored where female pufferfish keep their eggs. When she lays them, she also releases some of the toxin -- and it sticks to the babies once they hatch.
It's not enough to kill a predator, but the researchers found that fish who tried to eat the babies would soon spit them back out. The predators may have been getting that same tell-tale tingle that human restaurant patrons pay through the nose for. But unlike foolhardy humans, fish are smart enough to take that toxic tingle for what it is -- a warning.