The most common Irukandji, Carukia barnesi, are the size of a chickpea, and because they’re colorless, in the ocean they’re more or less invisible. The smaller ones might appear to you as the residue of a sneeze. The Irukandji’s translucent bell, shaped like a tiny boxing glove, trails four tentacles, delicate as cotton thread and about three feet long. The jellyfish’s sting doesn’t hurt overmuch. The pain is perhaps equivalent to a mild static zap from a metal doorknob—hardly even enough to make you want to suck your finger. The C. barnesi does not leave red welts, as other jellyfish do. You might miss the prick of its microscopic, stinging darts. You might think it’s just the start of sunburn.

Worst-case scenario: You’re dead by the following sunset. There are thought to be 25 species of Irukandji. One species, Malo kingi, is commonly known as “the king slayer.” After the initial sting comes a procession of ever more dreadful symptoms: back pain, agitation, the sensation of crawling skin, vomiting. The heart can become arrhythmic. Fluid may build up in and around the lungs. Patients “beg their doctors to kill them, just to get it over with,” the marine biologist Lisa-ann Gershwin told ABC Radio National in 2007.