Carukia barnesi

Irukandji jellyfish show why size says nothing about danger. Carukia barnesi, the species best known for it, is a tiny box jellyfish with a transparent bell only 1.5 to 2.5 cm across, and its sting causes only minor skin irritation at first. The trouble comes later. Some 5 to 40 minutes after the sting, Irukandji syndrome can set in, bringing generalized pain, often severely high blood pressure, nausea, vomiting and distress. Several small box jellyfish are suspected of causing it, but Carukia barnesi is the only one conclusively known to. Anyone stung in tropical Australian waters should have vinegar poured on the sting for at least 30 seconds and an ambulance called, even if the sting seems mild.