The blue-ringed octopus exists.
Blue-ringed octopus . Despite their small size , 12 to , and relatively docile nature , they are dangerous to humans if provoked and handled , because their venom contains tetrodotoxin , a neurotoxin powerful enough to kill humans . Typically , 50 -- 60 blue rings cover the dorsal and lateral surfaces of the mantle . They hunt small crabs , hermit crabs , shrimp , and other small crustaceans .