Jellyfish are a highly diverse group of aquatic animals. Their diameters range from 1 mm to over 1 m. Most have long tentacles, in some cases reaching more than 10 m in length. They come in a wide array of shapes and sizes. Some are bioluminescent; others are extremely poisonous. There are species that resemble delicate flowers rooted on the ocean floor but that turn out to be voracious predators when small crustaceans or fish larvae approach. What they all share, besides gelatinous bodies, is beauty and an air of mystery.