Box jellyfish, a class that includes 50 described species, have tentacles covered in tiny biological booby traps known as cnidocysts. Each cnidocyst contains a tiny dart and a load of poison that cause “the most explosive envenomation process that is presently known to humans,” according to a 1988 paper in The Medical Journal of Australia.
Consider Chironex fleckeri, a basketball-size box jelly found in the Indo-Pacific that dangles dozens of 6-foot-long, ribbonlike tentacles. Its nickname is Sea Wasp, and if one of these tentacles so much as brushes against your body, you may die before you reach the beach.
Still, a quick death might be preferable to what’s in store after a sting by other box jelly species, including the thimble-size Carukia barnesi.
In addition to severe headaches, vomiting, rapid heartbeat, and pulmonary edema, you may experience anxiety so powerful that some victims, convinced of their own doom, have reportedly begged their doctors to kill them and get it over with.
Scientists aren’t certain why some species of box jellyfish seem to produce more complicated and dangerous clinical syndromes than others. One explanation is that the venom of the dreaded Chironex is injected more deeply than, say, Hawaii’s Alatina.