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Carukia barnesi is a small and extremely venomous jellyfish found near Australia. Stings can result in Irukandji syndrome, and thus this species is commonly known as Irukandji jellyfish, although this name does not distinguish it from other Irukandji jellyfish such as Malo kingi.
A mature C. barnesi's bell is only 12 by 30 millimetres (0.47 by 1.18 in) in height. It has four contractile tentacles, one extending from each bottom "corner" of its bell, ranging in length from 50 to 500 millimetres (2.0 to 19.7 in).
The jellyfish was discovered by Jack Barnes, who, while on an exploration mission aimed at determining the reason for Irukandji syndrome, allowed himself to be stung by the jellyfish, while his 14-year-old son and a lifeguard looked on. The jellyfish was later named after him.