Blue-ringed octopuses are among the deadliest animals in the sea. Although they are about the size of a golf ball, they can pose a deadly threat to humans. When the octopus is agitated, the brown patches darken dramatically, and iridescent blue rings appear and pulsate. In a bite or even skin to skin contacts, this octopus passes on a deadly venom. Within five to ten minutes, the victim begins to experience numbness, progressive muscular weakness and difficulty breathing and swallowing. Death may result because of cerebral anoxia. (Source)