Octopuses, which are organisms of the class Cephalopoda in the phylum Mollusca, are generally harmless and unlikely to be aggressive unless provoked.
Their bites are rarely life threatening, except for the bite of the greater blue-ringed octopus, Hapalochlaena lunulata and the southern blue-ringed octopus (also known as the Australian spotted octopus) Hapalochlaena maculosa, which are found in coastal waters and tide pools around Australia and other Western Pacific tidal pools. [1, 2] A third species, the blue-lined octopus Hapalochlaena fasciata, has also been described. These octopuses grow up to 20 cm in length with tentacles extended. They are normally light-colored with dark brown bands and blue rings or patches. When disturbed, their bodies darken, and the blue circles turn iridescent blue. Their venom can be released into the water to paralyze their prey, but its effects on humans primarily occur by injection of the venom upon biting. [3, 4, 5, 6, 7, 8]
The blue-ringed octopus is shown below.
See Deadly Sea Envenomations, a Critical Images slideshow, to help make an accurate diagnosis.
There are many fractions in the venom secreted from the salivary glands of the blue-ringed octopus, one of which is identical to tetrodotoxin. This substance has also been found in multiple other tissues in H lunulata and H fasciata through fluorescent light microscopy.  Tetrodotoxin blocks voltage-gated fast sodium channel conduction, blocking peripheral nerve conduction, which can lead to paralysis and death from respiratory failure. Nerve conduction studies in tetrodotoxin-poisoned (puffer fish) persons have demonstrated reduced motor and sensory conduction velocities consistent with inhibition of sodium currents at the node of Ranvier.  Reported central nervous system effects of tetrodotoxin in humans have included nausea and emesis, miosis, diabetes insipidus, and depressed cortical activity.
Other fractions of the venom include 5-hydroxytryptamine, hyaluronidase, tyramine, histamine, tryptamine, octopamine, taurine, acetylcholine, and dopamine. 
The blue-ringed octopus does not naturally dwell in the coastal waters of the United States.
Rare cases of octopus envenomation occur in the Indo-Pacific region.
Individuals bitten by a blue-ringed octopus would have to be old enough and mobile enough to be able to walk or swim in the tide pools and coastal waters of Australia.
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