These guys definitely win for the least scary name, but the box jellyfish is proof that some of the most innocent-looking things can actually be incredibly dangerous — sort of like Justin Bieber. The fearsome box jellyfish or sea wasp can be found in the tropical Indo-Pacific, and their tentacles, each of which contain about 5,000 stinging cells, pack a serious punch. Box jellyfish poison is among the most toxic in the world, attacking the heart and nervous system. The pain from a sting is apparently so great victims beg to have the poisoned limbs amputated, and can sometimes simply go into heart failure because of the agony. Oh, and they're also nearly translucent, meaning swimmers can collide with them unaware. Your best bet might be to make friends with a sea turtle — they're impervious to the box jelly venom, and love to eat the spongy creatures.