Irukandji Jellyfish

Genuinely dangerous, though fatalities are rare relative to the number of stings. Most people stung develop severe Irukandji syndrome — hours of intense pain, vomiting, and a well-documented sense of impending death — that is not itself usually fatal but requires hospital pain management and monitoring, since blood pressure can spike dangerously and in a small number of cases has led to brain haemorrhage or heart failure. Confirmed and suspected Irukandji-related deaths in Australia are documented but uncommon, especially since awareness and medical response have improved; the greater practical risk is a large number of people needing emergency treatment for severe illness during stinger season.

For years, holiday-makers in parts of tropical Queensland were being hospitalised with a severe, mysterious illness after swimming in apparently jellyfish-free water, since no obvious sting mark or large jellyfish was ever found. In 1964, physician Jack Barnes suspected a small, nearly invisible jellyfish was responsible, caught a tiny specimen, and to prove it deliberately let it sting himself, a nearby lifeguard, and, controversially by modern standards, his own nine-year-old son, then documented all three developing the full syndrome under medical observation. This confirmed the link and the jellyfish was later named Carukia barnesi in his honour.

Its small size and near-total transparency are the main reasons. An Irukandji jellyfish's bell may be barely a centimetre or two across, and both the bell and the fine trailing tentacles are close to invisible in clear tropical water, especially against sunlight glare or waves, so swimmers typically have no warning before being stung. The animal is also small enough in some cases to pass through the mesh of standard box-jellyfish exclusion nets set up at beaches, which is why full-body lycra "stinger suits," rather than nets alone, are the standard recommended protection during the Australian stinger season, roughly November to May in affected areas.

Because it is often impossible to tell at the time whether a sting is from a relatively minor species or something capable of causing Irukandji syndrome or the more acutely dangerous box jellyfish sting, official first aid in these waters follows a specific protocol — get out of the water, call for emergency help, pour vinegar liberally over the sting site for at least 30 seconds to deactivate any unfired stinging cells still on the skin, and keep the person still and monitored, since Irukandji symptoms can take up to 40 minutes to develop. Anyone who develops severe pain, breathing difficulty, or the described sense of dread after any sting in these waters should be taken to hospital rather than treated at home.