The Irukandji Jellyfish, Carukia barnesi, is a species of jellyfish. It lives in the waters of northern Australia. It is a very deadly jellyfish. The Irukandji is only 2.5 centimeters in diameter. This makes it hard to find. The Irukandji is believed to be the most poisonous creature in the world. Some people are not sure if that is true.