A lion's mane jellyfish in Narragansett Bay. Notice that many smaller jellies are captured in this jellyfish's tangle of tentacles. During the summer of 2008, the lion's mane jellyfish, which has a very painful sting, swarmed along the east coast of the U.S. The largest lion's manes live in the Arctic and may have tentacles that are up to 100 feet long. That's longer than a 100-foot-long blue whale.