Allow me to introduce the blue-ringed octopus. If you were expecting it to look like a monstrous scourge of the Indo-Pacific reefs, rampaging through the shallow seas and striking fear into all who encounter it, you would be mistaken. The blue-ringed octopus is small (only about 5-8 inches total) and is entirely adorable. Its typically kind of boring-looking, camouflaging with its surroundings like many octopus do, but when it feels threatened, it flashes iridescent blue spots (hence the name) (1). There are four known species of these octopods, all ordered within the genus Hapalochlaena.
It might surprise you to know that all octopus have venom, but these are the only ones that could be deadly. Typical octopus venoms are produced by the animal itself at the mouth (beak) to more effectively capture prey, and the bite is entirely harmless to most humans. The venom of the blue-ringed octopus, however, is especially potent not by their own making, but because of the bacteria that they harbor and form a symbiotic relationship with. These bacteria produce tetrodotoxin (TTX), a neurotoxin that is famously found in pufferfish. Because another species produces the toxin for the octopus, it is called exogenous (outside origin) rather than their own venom that all octopus produce which would be called endogenous (inside origin). Blue-ringed octopus are the only known octopus species to produce venom that is both.
The octopus’ diet consists mostly of crabs and shrimp, so it is not known why they have develop enough venom to kill 26 adult humans within minutes of a bite (2). They use the venom for both defense and predation, but only attack people when threatened or stepped on. Even when people are bitten, it is not often actually lethal. Seeking quick medical attention can usually save the life of the diver, as long as they are aware that they have been bitten and leave the water immediately.
Scientists think that blue-ringed octopods have developed this lethality mostly for defense, based on the fact that it can be found in almost all body tissues and even the ink, unlike other octopus who only have venom in their beaks (3).
The bacteria that produce the TTX are concentrated in the salivary glands, from which scientists have been able to extract dozens of bacterial strains that they cultured to produce TTX (or related compounds). They hypothesize that the salivary glands send the TTX through the intestinal blood stream, to distribute it to the rest of the body (4). The brachial hearts (both of them!) might also be used to pump the venom via the gills. Naturally, the blue-ringed octopus is immune to its own venom and doesn’t suffer the usual side-effects of TTX (5).
All About TTX
Tetrodotoxin (TTX) is a "natural" toxin, meaning that it always has a biological origin. That doesn't necessarily mean that all of the animals that use TTX in their toxin arsenal produce it themselves. Most often, they rely on bacteria to do it for them. Just like the bacteria in our guts that help us to digest food we would otherwise not be able to eat, many animals form partnerships with bacteria and rely on them for all manner of physiological systems. Using bacteria as a means of defense or hunting might seem unusual, but this is only one example. Sponges have bacteria that help reduce free-radicals from damaging their tissues, some shrimp have bacterial buddies to fend of harmful fungi, and just about every animal out there has a particular microbiome that is carefully balanced to keep us healthy (6). TTX itself is also relatively common among the animals. Snails, newts, frogs, fish, and crabs share the blue-ringed octopus' TTX superpower (7). It's a powerful toxin that specifically targets sodium channels in the body, meaning that nerve cells are unable to send messages.
It's important to consider this if you ever have the inclination to nibble one of the species who produce it, as TTX is both water soluble and heat stable. Therefore, unlike many harmful compounds, cooking doesn't remove it. As a general rule, it's best to avoid the pufferfish on the menu unless you have a very capable chef.
Toxins: Venom, Poison, or Toxungen?
So what exactly separates one type of toxin from the other?
Basically, the same compound can be any of the three depending on how it is administered.
A venom must be injected, as with a snake or spider bite. A poison must be ingested or eaten, like a toad or dart frog. A toxungen is the name for a toxin that is secreted and delivered without a wound but not eaten, like a spitting cobra (8).
All octopus are venomous by nature of their bite, but only the blue-ringed octopods are venomous, poisonous, AND toxungenous! They inject venom through a bite, but because they also store the TTX in tissues, they are deadly if eaten, and because they also store TTX in their ink, spraying it as a defensive mechanism, they are toxungenous. The ink sac, however, is mostly vestigial (underdeveloped and not used) in adults, but juveniles retain the ability to emit ink (9).
So blue-ringed octopus might not be as terrifying at first sight as their colossal squid cousins, but they have earned a killer reputation despite their cute image.