I just found out from The Cephalopod Page that some octopuses have a venom that can quickly kill humans:
Typically, the victim is unaware of the danger and either picks up the innocuous looking octopus or inadvertently contacts it. The bite is slight and produces at most only a small laceration with no more than a tiny drop of blood and little or no discoloration. Bites are usually reported as being painless. Often the victim doesn’t even know that he had been bitten. This can make it difficult for emergency and medical personnel to determine the cause of a patient’s distress. In fact, there is some question as to whether the octopus even needs to bite to envenomate a human. In cases with prolonged contact, the venom might pass directly through the skin. While most severe envenomations appear to involve bites, I can report developing mild local neurological symptoms after immersing my hand in sea water in which a large blue-ring had been shipped.
Seems like powerful stuff. Probably most dangerous if you try to eat or drink the toxin. The damage potential of even a small octopus is impressive:
The toxin was characterized as a low molecular weight, non-protein molecule and was named maculotoxin. It was recognized to be similar to tetrodotoxin (TTX), the extremely deadly toxin found in pufferfishes Experiments with rabbits showed that a single adult blue-ringed octopus weighing just 25 g possessed enough venom to fatally paralyze 10 large humans.
Their salivary glands harbor dense colonies of TTX-producing bacteria. The blue-rings have evolved a symbiotic relationship with the bacteria, providing them ideal living conditions while using the toxin they produce to subdue prey and as part of their highly advertised defense.
There’s a beautiful picture of one available HERE, probably taken just before it killed the photographer, William Tan.