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.
However, note that basically the same information is contained in a 2002 "molecule of the month" post by Jim Johnson.
Separately, from the Nobel Prize presentation speech in chemistry (1965):
Professor Woodward's activity has by no means been restricted to synthetic work. He has established the structure of many important compounds, for instance the peculiar fish poison tetrodotoxin, causing numerous fatalities in Japan
Also, Professor Harry Mosher at Stanford did much work on tetrodotoxin.
See also U.S. Patent 6,552,191, Method of extracting tetrodotoxin and U.S. Patent 7,576,202, Tritium-traced saxitoxin dihydrochloride and method for the production thereof.