The Irukandji jellyfish (/ˌɪrəˈkændʒi/ IRR-ə-KAN-jee) are any of several similar, extremely venomous species of rare box jellyfish. With a very small adult size of about a cubic centimetre (1 cm or 0.061 in), they are both one of the smallest and one of the most venomous jellyfish in the world. They inhabit the northern marine waters of Australia, and cost the Australian government $AUD 3 billion annually through tourism losses and medical costs associated with stings. This type of jellyfish reproduces sexually with eggs and sperm. They fire their stingers into their victim, causing a condition known in humans as Irukandji syndrome, which can be fatal and difficult to immediately recognise due to the delayed effects of the venom. There are about 16 known species of Irukandji, of which Carukia barnesi, Malo kingi, Malo maxima, Malo filipina and Malo bella are the best known.
Irukandji syndrome was named in 1952 by Hugo Flecker, who first described the symptoms of envenomation by this jellyfish. The syndrome was named after the Irukandji people, whose region stretches along the coastal strip north of Cairns, Queensland. The first of these jellyfish, Carukia barnesi, was identified in 1964 by Jack Barnes; to prove it was the cause of Irukandji syndrome, he captured the tiny jellyfish and allowed it to sting him, his nine-year-old son, and a robust young lifeguard. They all became seriously ill, but survived. Australian toxicologist Jamie Seymour made a documentary about the jellyfish called Killer Jellyfish.
