The largest blue-ringed octopus is no bigger than a golf ball, but these miniature cephalopods don't like to be messed with. Their venom is one of the strongest in the world, and they use it to kill their enemies very quickly. At rest, their brown bodies blend in with their surroundings. When alarmed, their glowing blue rings tell predators to stay back or risk death if they don't. Incredible photographs of blue-ringed octopuses in their natural habitat will amaze readers of any age.