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Note: Bret Grasse, С����Ƶ Manager of Cephalopod Operations, provides commentary.

Octopuses have gotten a reputation for being cunning camouflagers and intelligent creatures. But some are known for a more ominous reason: They're deadly.

One group — the blue-ringed octopus (genus Hapalochlaena) — is especially dangerous. But what makes this creature so lethal?

Blue-ringed octopuses carry a killer concoction called tetrodotoxin (TTX), a potent neurotoxin that can paralyze living things, including humans. Tetrodotoxin is most famously known from pufferfish — improperly prepared "fugu" can kill those who eat it. Tetrodotoxin is 1,200 times more toxic to humans than cyanide and it has no known antidote.

All four of the octopus species in the genus Hapalochlaena have tetrodotoxin, making them the four most venomous species of octopus: the greater blue-ringed octopus (H. lunulata), the southern or lesser blue-ringed octopus (H. maculosa), the blue-lined octopus (H. fasciata) and the common blue-ringed octopus (H. nierstraszi). These cephalopods, with their iridescent blue circles, are indeed beautiful, but also dangerous. .