Close-up of the bell of a box jellyfish (Cubozoa). Box jellyfish are relatives of true jellyfish, but have a cubic bell and tentacles extending from each lower corner. Unlike true jellyfish they have complex eyes and are active swimmers, avoiding obstacles and chasing after prey. They feed mainly on small fish, which they catch with their stinging tentacles. Some species of box jellyfish have a sting that is rapidly fatal to humans. Filmed in the Gulf of Thailand, near Koh Pha Ngan Island.