A lion's mane jellyfish, Cyanea capillata, off the coast of New Jersey, USA. This jellyfish has a bell reaching up to eight feet in diameter, and tentacles longer than a blue whale - up to 200 feet long. Juveniles are pink, turning red as they mature, becoming brownish purple as adults. This jellyfish can cause severe stings, and there have been rare reports of fatalities to swimmers. In one of the Sherlock Holmes mysteries by Sir Arthur Conan Doyle, a lion's mane jellyfish is the killer. It is found from temperate waters to the Arctic.