The blue-ringed octopus is a small, venomous cephalopod known for the iridescent blue rings that appear when it's threatened, signaling its potent neurotoxin, tetrodotoxin, which can cause paralysis and death in humans. Found in Pacific tide pools from Japan to Australia, these octopuses are generally shy but deliver a bite that is often painless, with symptoms like numbness and breathing difficulty appearing later. There is no antivenom, so immediate medical attention, including CPR, is critical for bites.