Tetrodotoxin is found in more than just marine creatures. It is present in high concentrations in the skin of certain newts in North America and Japan, and in several kinds of frogs in Central and South America and Bangladesh. The widespread occurrence of tetrodotoxin poses some intriguing riddles. First, how is it that such different animals, belonging to separate branches of the animal kingdom, have all come to possess the same deadly poison? And how is it that they are able to tolerate high levels of tetrodotoxin while others cannot?

The questions are particularly interesting because, in general, animal toxins are distinct and specific to each group. For instance, the venoms produced by snakes and scorpions are made of different kinds of toxins. But the tetrodotoxin found in each dish of that deadly buffet is identical.

One explanation could be that each of these animals has independently found a way to synthesize tetrodotoxin. But the toxin is a rather complex molecule that requires several chemical steps to assemble. It seems very unlikely that the molecule would be invented many times over in different animals. Rather, the evidence suggests that animals do not make the toxin themselves.

For instance, when puffer fish are raised in aquariums with filtered, bacteria-free water, they are nontoxic. Similarly, when Japanese newts or Panamanian frogs are raised on special diets, they lose their toxicity. These experiments indicate that tetrodotoxin-bearing animals obtain the toxin from the food chain. Indeed, several species of tetrodotoxin-producing bacteria have now been isolated from puffer fish, the blue-ringed octopus, certain snails and other animals. It appears that the animals become toxic by sequestering the bacterially produced toxin in their tissues.

While those discoveries solve the mystery of the source of tetrodotoxin, they do not quite explain how so many kinds of animals exploit it. Tetrodotoxin attacks an ancient feature of the animal kingdom, blocking channels that normally control the movement of sodium ions across nerve and muscle cell membranes and halting their electrical activity. All animals have these sodium ion channels, and the part of the channel that tetrodotoxin fits into and blocks is generally very similar among them.