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. The same bacterial origin explains why unrelated animals in different oceans carry an identical compound. Tetrodotoxin-resistant sodium channels in the octopus keep it 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 is metabolised, and patients given prompt artificial ventilation until it clears usually recover fully.