The Irukandji is a box jellyfish with a transparent bell five to 25 millimeters wide, about the size of a sugar cube, trailing four tentacles that can reach a full meter. This episode examines how something so small and fragile carries venom capable of putting a healthy adult in intensive care. Its stinging cells fire like pressurized harpoons, and unlike most jellyfish it carries them on the bell as well as the tentacles. Research from North Queensland in 2015 suggested why: the Irukandji is not a passive drifter. It hunts small, fast fish and needs venom strong enough to stun prey before the struggle tears it apart.

The human story is just as strange. Hugo Flecker named Irukandji syndrome in 1952 without ever finding its cause, and in 1964 toxicologist Jack Barnes proved the culprit by letting one sting himself, his nine-year-old son, and a lifeguard. The sting itself feels minor. Then, about 30 minutes later, the body floods with stress hormones, bringing violent cramps, soaring blood pressure, and a feeling of impending doom. There is still no antivenom, and experts cannot agree on first aid.

• About 16 species are known, and the syndrome takes its name from the Irukandji people of the coast north of Cairns.
• Because the reaction is delayed and leaves almost no mark, deaths have likely been misattributed to heart attacks or drownings.
• The Australian Resuscitation Council recommends vinegar on the sting, while some researchers argue it can trigger unfired stingers or push venom deeper.
• Hospital care is purely symptomatic: antihistamines, blood pressure drugs, intravenous opioids, and magnesium sulfate, which helps some patients and does nothing for others.
• Warming water is carrying the jellyfish south and west. Reported stings around Great Palm Island rose from 12 in 2019 to 23 by early December 2020.