Which venomous marine animal's sting causes Irukandji syndrome?

The Irukandji jellyfish is the venomous marine animal whose sting causes Irukandji syndrome. The name covers several tiny box jellyfish, especially Carukia barnesi and Malo kingi, found mainly in northern Australian waters.

The syndrome was named in 1952 by Australian physician Hugo Flecker after the Irukandji people of coastal Queensland. In 1961, physician Jack Barnes linked the illness to a small box jellyfish by allowing one to sting him, his nine-year-old son, and a lifeguard. All three developed the characteristic symptoms but survived.

The initial sting may feel like little more than a mosquito bite, yet severe symptoms can appear 5–120 minutes later. Victims may develop intense muscle and back pain, sweating, vomiting, anxiety, rapid heartbeat, and dangerous hypertension. Unlike the Portuguese man o’ war or a typical sea nettle, Irukandji jellyfish are cubozoans, and the syndrome can be caused by more than one species.