After Hours

Jarring jellies

What species of jellyfish produces the most potent venom?


We asked what species of jellyfish produces the most deadly venom, one that's even more dangerous than that found in the Portuguese Man-of-War.

The Indo-Pacific box jellyfish, also known as the Sea Wasp or Chironex fleckeri, is among the most deadly sea creatures. The average mature box jellyfish possesses enough toxin in its nematocysts to kill as many as 60 average-size humans, and its venom has been known to slay victims in less than five minutes.

Native to the coasts of Australia, the box jellyfish has a box-shaped upper bell that's about the size of a person's head, and it boasts tentacles up to three meters long. The box jellyfish's venom attacks the cardiovascular system, often inducing cardiac arrest if antivenin isn't administered almost immediately.

Despite its ability to inject this deadly poison, scientists estimate that box jellyfish have killed only an average of one person per year for the last century. This surprising statistic is largely due to the fact that these sea creatures are frightened by the motions of objects or creatures larger than themselves and flee from humans if given sufficient warning. Box jellyfish congregate near tidal estuaries, rather than reef areas or open seas, which may also limit lethal encounters with swimmers.

It's safe to say that box jellyfish attacks are completely unintentional. Box jellyfish have no brains; their sensory eyes tie together into a nerve cluster that reacts entirely by reflex. Thus, the old adage about wasps also applies to Sea Wasps: If you leave it alone, it will do the same for you.

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