Stung! On Jellyfish Blooms and the Future of the Ocean by Lisa-ann Gershwin, with a foreword by Sylvia Earle University of Chicago Press, 424 pp., $27.50

It’s become fashionable to keep jellyfish in aquariums. Behind glass they can be hypnotically beautiful and immensely relaxing to watch. Unless we are enjoying them in this way, we usually give little thought to the creatures until we are stung by one. Jellyfish stings are often not much more than a painful interlude in a seaside holiday—unless you happen to live in northern Australia. There, you might be stung by the most venomous creature on Earth: the box jellyfish, Chironex fleckeri.

Box jellyfish have bells (the disc-shaped “head”) around a foot across, behind which trail up to 550 feet of tentacles. It’s the tentacles that contain the stinging cells, and if just six yards of tentacle contact your skin, you have, on average, four minutes to live—though you might die in just two. Seventy-six fatalities have been recorded in Australia since 1884, and many more may have gone misdiagnosed or unreported.

In 2000 a somewhat less venomous species of box jellyfish, which lives further south, threatened the Sydney Olympics. It began swarming at the exact location scheduled for the aquatic leg of the triathlon events. The Olympic Committee considered many options, including literally sweeping the course free of the menace, but all were deemed impractical. Then, around a week before the opening ceremony, the jellyfish vanished as mysteriously as they had appeared.

Most jellyfish are little more than gelatinous bags containing digestive organs and gonads, drifting at the whim of the current. But box jellyfish are different. They are active hunters of medium-sized fish and crustaceans, and can move at up to twenty-one feet per minute. They are also the only jellyfish with eyes that are quite sophisticated, containing retinas, corneas, and lenses. And they have brains, which are capable of learning, memory, and guiding complex behaviors.

The Irukandjis are diminutive relatives of the box jellies. First described in 1967, most of the dozen known species are peanut- to thumb-sized. The name comes from a North Queensland Aboriginal language, the speakers of which have known for millennia how deadly these minuscule beings can be. Europeans first learned of them in 1964 when Dr. Jack Barnes, who was trying to track down the origin of symptoms suffered by swimmers in Queensland, allowed himself to be stung by one. With nobody attending but a lifeguard and his fourteen-year-old son, he was lucky to survive.

It’s now known that the brush of a single tentacle is enough to induce “Irukandji syndrome.” It sets in twenty to thirty minutes after a sting so minor it leaves no mark, and is often not…