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.

That desperation is often accompanied by one of the more striking indications of contact with an Irukandji jellyfish: a sense of impending doom. To the afflicted person, nothing seems likely to alleviate distress, no medical professional offers hope. The swimmer might not have seen or felt the sting, but if a touch point can be identified, the treatment is to splash the area with vinegar to neutralize any nematocyst cells on the skin’s surface. Then, if the malady progresses, morphine and antihypertensive drugs are administered. Very few people stung by an Irukandji will be so unlucky as to die, but at least one victim has compared the latter phases of envenomation to childbirth.

There may be as many as 4,800 different species of jellyfish. Not every kind possesses a sting that is perceptible to humans. Individual jellyfish are fragile creatures. Being composed largely of soft collagen, they easily tear. In a net or bunted along a reef by a storm surge, jellyfish are soon shredded. Washed ashore, they evaporate, leaving only a remnant halo of mesoglea (the jellyfish’s gluey core). Organized water: That was one 19th-century naturalist’s minifying description of the jellyfish. The creature’s wispy anatomy confers on it the specific beauty of the readily destroyed, a quality that elicits comparisons to things that are empty and lambent—light bulbs, dropped lingerie, a nebular constellation, the cellophane wrappers from hotel soaps, dribbles of wax.

How appealing it is to fashion metaphors out of a jellyfish. The animal is all stimulus, sensuousness without consciousness. Such evanescent creatures pose none of the anthropomorphizing complications of, say, octopuses. An octopus will regard you with features that resemble a face, and an intelligence that we’ve been advised is akin to that of dogs and dolphins. Most jellyfish are see-through, so we can tell they don’t have minds of their own to speak of. Eyeless, bloodless, brainless—jellyfish are more than alien enough to comfortably objectify.

Their delicacy notwithstanding, in recent decades jellyfish species have come to be thought of as the durable and opportunistic inheritors of our imperiled seas. Jellyfish blooms—the intermittent, and now widely reported, flourishing of vast swarms—are held by many to augur the depletion of marine biomes; they are seen as a signal that the oceans have been overwarmed, overfished, acidified, and befouled. These invasions are sometimes discussed as if they had the potential to culminate in ecophagy, the devouring of an ecosystem in gross. (Phage derives from the ancient Greek word meaning “to eat up.”) The vision—hat tipped to science fiction—is of the planet’s oceans transformed into something like an aspic terrine. In waters thickened by the gummy mucus of living and dead jellyfish, other sea life will be smothered. Because jellyfish recall the capsules of single-celled protozoa, this eventuality invites portrayal as a devolution of the marine world—a reversion to the “primordial soup.”