Carukia barnesi is minute, with a bell around one to two centimetres and four trailing tentacles. Its stinging cells are concentrated in clusters on the tentacles and also on the bell itself, a pattern unusual among cubozoans and one reason a light brush can deliver a dose.
The skin reaction is mild, sometimes just a faint mark. The systemic phase follows after a delay typically given as twenty to forty minutes: severe lower back pain, muscle cramps, nausea, sweating, restlessness and hypertension. The syndrome is understood to involve a large release of catecholamines, which accounts for the blood pressure rise and the sense of impending doom patients frequently report.
The species was pinned down in 1961 by Jack Barnes, who caught a specimen and applied it to himself, his son and a lifeguard to confirm it caused the syndrome. The common oversimplification is that Irukandji syndrome means one species. At least several cubozoans, including Malo kingi, produce comparable illness, and the syndrome is a clinical description rather than a species identification.