Hi Henry,
THis is from a response I wrote some time ago to a similar question:
"First of all, lets look at that word "poisonous". Although we use this word loosly in the US, it really means "poisonous when eaten". I imagine you are really asking, are octopuses "venomous". That is, do they have venom that they can somehow inject into another animal, paralyzing or killing it?
The answer is this: all octopuses are venomous. Their saliva contains a mixture of strong neurotoxins (venom) used to immobilize their prey. They use their beak and/or tongue to make a wound into which the toxic saliva can be squirted.
The amount and type of toxins in the saliva varies. The most venomous we know of are the Blue-ringed octopuses, found around Australia and the eastern Indo-Pacific. These have bacteria in their salivary glands that produce a powerful nerve toxin, tetrodotoxin (TTX). This toxin paralyzes the victim, who remains conscious but loses control of voluntary muscles and will die from lack of oxygen if not given respiratory support. Several human deaths have been attributed to bites from this small, attractive octopus, and there have been many near fatalities. "
Bimacs don't have this highly venomous toxin - many of our octo keepers have been bitten by young bimacs and report that it's something like a bee or a wasp sting. Have a look at the Journals and Photos forum: The Agent Bites and Inklet biting (mid May) These bites don't leave much of a mark, either. Of course, anyone might be allergic to the toxin, just as some people are allergic to bee stings, and you would have a more serious reaction. We haven't encountered this yet amoung any of our octo keepers.
Nancy