Every month we showcase a relationship between two or more species or groups of species that can be considered a mutualism: a relationship where both members benefit. This month’s mutualism is between tetrodotoxin-producing bacteria and their many hosts.
Until recently, the origin of the tetrodotoxin (TTX) in pufferfish was unknown. TTX was first isolated by chemists from the ovaries of pufferfish in 1950. It was thought to be a toxin found exclusively within pufferfish (hence its naming after the pufferfish family Tetraodontidae), until it was found in the California newt Taricha torosa. After its discovery in newts, TTX was rapidly isolated from a diversity of animal species completely unrelated to each other, including several frogs, an octopus, several marine snails, a crab, and a starfish. This raised an interesting question: did all these animals evolve the ability to produce tetrodotoxin separately or is there some common factor?
After some time, two scientists postulated that the TTX-containing animals were “infected” by unknown TTX-producing microorganisms living within their bodies, even though this had never been observed before. Turns out, they were right!
Many species of bacteria have been found to produce TTX, and several of these bacteria occur within animals. TTX-producing bacteria are maintained within the skin, gut, and ovaries of certain pufferfish, the salivary gland of the blue-ringed octopus, the digestive glands and muscles of several marine snails, and the spine muscles of a sea urchin. The TTX produced by the bacteria are used by the animals both defensively as a venom or poison (venom is injected while poisons are ingested) and offensively on their prey as a paralyzer. In return for providing a potent toxin, bacteria are allowed to live in a high-nutrient, low-competition environment, free of many of the stresses a bacteria has to deal with.
Pufferfish are generally believed to be one of the most poisonous vertebrates in the world, containing high levels of TTX. The meat of some species of pufferfish is considered a delicacy by some Asian cultures and is served by specially trained chefs who know which parts of the fish are safe to eat and in what quantity; these chefs are trained for two years before they are allowed to serve pufferfish in a restaurant.
Symptoms of ingesting TTX occur within 30 minutes in humans. At low levels, symptoms include tingling of the lips and tongue, followed by hypersalivation, headaches, and lethargy; higher dosages induce paralysis, seizures, and death. About five people die each year from TTX-poisoning. TTX blocks electrical impulses in nerves by binding to proteins that are essential in nerve cell firing. By binding to these proteins, TTX prevents the neurons from communicating with muscles, causing complete paralysis. Paralysis leads to death when essential muscles such as the heart and diaphragm (the muscle that pulls in air) stop working properly.
Don’t forget to check out the previous installments of mutualism of the month below and stay hungry!