It measures between 5 to 12 centimeters long. Despite its size, the blue-ringed octopus (Hapalochlaena maculosa) is the world's most dangerous octopus and one of the most redoubted living being. Only a bite is enough to kill one human. Its venom is composed of tetrodoxin, a neurotoxin that leads to severe breathing distress for the victim. The blue color of the rings gets bluer when it feels threatened, thus warning of the danger. The blue-ringed octopus lives in the Pacific Ocean.