Tetrodotoxin blocks voltage-gated sodium channels, halting nerve signalling and, at sufficient dose, paralysing the muscles used to breathe. Blue-ringed octopuses carry it in their posterior salivary glands and deliver it in saliva while biting prey such as small crabs.
The octopus is not the chemist. Bacteria, including Vibrio and related genera, cultured from the glands produce tetrodotoxin, and the animal appears to host them rather than build the molecule itself. Bacterial symbionts are also the leading account for pufferfish and other marine carriers. The picture is not uniform: in rough-skinned newts, captive animals raised on toxin-free diets keep producing tetrodotoxin, and no bacterial source has been isolated. Tetrodotoxin-resistant sodium channels keep the octopus from poisoning itself.
The blue rings are a warning, not a weapon: the animal flashes them by contracting muscles that expose iridophore-lined pockets, in well under a second. A persistent misreport is that the bite is invariably fatal. There is no antivenom, but the toxin clears from the body, and patients given prompt artificial ventilation until it does usually recover fully.