The dangerous part of the venom of blue-ringed octopuses is the neurotoxin tetrodotoxin. It works by blocking the passage of certain ions, thereby preventing neurons from carrying messages and thus muscles from flexing in response to environmental stimuli (i.e., something seen, felt, heard, etc.) This toxin is not produced by the octopus itself though; bacteria living in the salivary glands produce the toxin. The octopi and bacteria have a symbiotic relationship, meaning that each benefits from the presence of the other (the octopi get a potent weapon, and the bacteria get a nice place to live).

Tetrodotoxin is used by several other organisms that host the same or similar bacterial colonies, with pufferfish that are not prepared properly being a common source of tetrodotoxin poisonings.

The poison is tetrodotoxin, which paralyses voluntary muscles (which includes the muscles that you use to breathe, which is why it's so deadly).

Blue ringed octopus venom is made of a neurotoxin (a substance which is poisonous to the brain) called tetrodotoxin. It is the same toxin found in puffer fish. It is produced by bacteria in the salivary glands of the octopus. The venom blocks nerve signals in your body. The effects of the poison start with muscle numbness, and eventually escalate to blindness, loss of senses, loss of motor skills, and eventually muscle paralysis - which leads to respiratory arrest since we need muscles to breathe. But don’t worry, the blue ringed octopus is not known to be aggressive. There have been no reported deaths of blue ringed octopus bites since the 1960s!