Greater blue-ringed octopus (Hapalochlaena lunulata) Although potentially deadly when disturbed, blue-ringed octopuses, such as this tiny creature, are nonaggressive and reclusive, preferring to remain concealed in rock crevices, empty mollusk shells, or even discarded bottles. (more) blue-ringed octopus mollusk Also known as: Hapalochlaena Written by Manjishtha Bhattacharyya Manjishtha Bhattacharyya is an editor at Encyclopaedia Britannica. She holds a PhD in zoology from Vidyasagar University, West Bengal, and has more than seven years of experience in science communication... Manjishtha Bhattacharyya Fact-checked by Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Britannica Editors Last updated Mar. 6, 2026 •History blue-ringed octopus, (genus Hapalochlaena), any of several Indo-Pacific octopus species known for their potent venom and their ability, when disturbed, to signal danger by flashing vivid blue rings or stripes in their skin. They can deliver a bite injecting the neurotoxin tetrodotoxin (TTX), which may be fatal. Although several species of these cephalopods have been proposed, only four are currently recognized—Hapalochlaena lunulata, H. maculosa, H. fasciata, and H. nierstraszi—each showing a region-specific distribution. Taxonomy Species...

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