Sea Wasp Box Jellyfish (Wikimedia) This is the most deadly jellyfish in the world, and one of the most deadly creatures on the planet altogether. It is also known as the marine stinger. Since 1954, 5,568 deaths have been attributed to this jelly. There fifteen tentacles on this type of jellyfish, which may extend ten feet in length. On each tentacle, there are around half a million darts. These microscopic darts are full of venom. Each one could theoretically kill up to 60 people.