In the Sherlock Holmes story “The Adventure of the Lion’s Mane,” our hero is strolling along a beach when he comes across a man in his death throes, staggering and screaming before shouting his last words: “The lion’s mane!” His name is Fitzroy McPherson, and all over his back are thin red lines—which Sherlock notices because he's a detective and all—as though the man "had been terribly flogged by a thin wire scourge.”

McPherson’s colleague, a mercurial fellow named Ian Murdoch, becomes a person of interest. He had, after all, once thrown McPherson’s dog through a plate glass window. But that suspicion falls to pieces when the dog-hurler himself staggers into Sherlock’s home in comparable agony, all marked up with the same red lines.

And then the answer hits the great detective. With a police inspector and a guy named Stackhurst he hurries to the beach and finds the culprit: “Cyanea!” he cries. “Cyanea! Behold the Lion’s Mane!” It’s a great jellyfish among the rocks. Shouts Sherlock: “It has done mischief enough. Its day is over! Help me, Stackhurst! Let us end the murderer forever.” And with that they push a boulder into the water, crushing the critter.

That’s a whole lot of animal cruelty in a single short story, and the severity of a sting from a lion’s mane jellyfish, known scientifically as Cyanea capillata, is highly exaggerated here. But this critter is actually far more remarkable than its fanciful villainization. What Sherlock failed to mention is that this is the world’s largest jellyfish, with a bell that reaches a staggering 8 feet wide and tentacles that grow to 120 feet long, far longer than a blue whale. And this monster is really, really loving the whole global warming thing, conquering more and more of Earth's oceans in massive blooms. So please, if you will, welcome our new giant gelatinous overlords.

"Hey, kids? Do me a solid and keep your hands off of me." Photo: Richard Lim

It’s those seemingly endless tentacles, hundreds and hundreds of them, that make this incredible growth possible, according to Lisa-Ann Gershwin, a marine biologist with Australia’s Commonwealth Scientific and Industrial Research Organisation. “They've got all of these fishing lures out there at the same time,” she said. “Every single tentacle is out there to catch something. They can find so much food simply by multitasking, really.”

Lion’s manes will take just about anything, from the tiniest zooplankton—little critters and fish larvae and such that drift in the open ocean—to smaller jelly species and even their own kind. Their mighty weapons are stinging cells known as nematocysts, which on contact fire poisonous barbs into the prey (think Scorpion from Mortal Kombat, only nematocysts didn't used to get me in trouble for spending so much money in arcades).

Though nowhere near as powerful of the notoriously deadly box jellyfish, the sting of the lion's mane is more than enough to incapacitate small critters—and dish out searing pain to humans. (Gershwin herself once had a lion's mane sting her foot, which "went all red and puffy" and felt like it was being stabbed with "thousands of needles.") Thoroughly ensnared by the tentacle's innumerable spines and none too healthy on account of the poison, the prey is reeled in. The lion’s mane can do this a single tentacle at a time, contracting the muscles in each until the prey reaches its curtain-like “oral arms,” folds of tissue in its bell.

The photographer likely suffered greatly to take this picture, but rest easy knowing that we paid for the rights to republish it, so at least he's getting compensated for his efforts. Photo: Cultura Science/Alexander Semenov/Getty Images

From here the prey passes into the jelly’s mouth, which is really just a hole in its body that also functions as its anus, and finally moves into the stomach. “And then they have a circulatory system of canals where the nutrients from the stomach are just dispersed out to the rest of the body through this network,” said Gershwin. “It’s really, really simple, but it works really well. I mean, they've been doing exactly that for 600 million years, and it works so well they haven't needed to change it.”

That's quite an evolutionary sweet spot. Such a sweet spot, in fact, that the lion's mane never bothered to evolve true eyes. Instead, these jellies have extremely rudimentary eyespots and can do nothing more than detect light and dark—no shapes and certainly no colors (interestingly, box jellyfish have eyes more like our own, complete with lenses and such, presumably so they can observe the terror they strike in humans). And a brain? Not really necessary, as it turns out. They do have nerve bundles that essentially automate all of their processes, but these are nothing like a brain as we would recognize it.

“A brain is kinda overrated, really,” said Gershwin. “We find it kind of entertaining, and a little bit important, but they do all the stuff they need to do without a brain. But so do venus fly traps. Lots of things can actually do kind of sophisticated behaviors without a brain.”

Reproduction for the lion’s mane, though, is quite sophisticated. Males release sperm threads into the water, and females hoover them up with their mouth-anus thing, a totally unscientific term that I just made up. Her eggs are fertilized internally, and when they hatch, the larvae roam around a bit inside her, then drift off to settle on the seafloor.

But these larvae don’t turn right into what we would identify as jellies, in what is known as the medusa stage, named after the mythical lady with snakes for hair. Instead, they become little white tubes with frilly ends called polyps, which wait until conditions are just right to actually clone themselves hundreds of times over, releasing baby jellies into the water column. Though scientists have yet to do genetic testing on this, Gershwin suspects that huge blooms of lion’s mane jellies could in fact all be clones from a single tiny polyp. It's a bit like Attack of the Clones, only interesting.

Sting Operation

And boy have they been blooming. Populations of jellyfish like the lion’s mane seem to be exploding in the world’s oceans—because, bluntly put, we've goofed. Global warming, overfishing, pollution, basically anything terrible we’ve done to the seas have been an absolute boon to jellyfish, according to Gershwin. Data on jellyfish populations is scarce, so nothing is yet definitive, but as Gershwin puts it, “we now find ourselves in the unexpected position of knowing that we have serious problems with stings to tourists and cloggings of power plants and salmon kills and whatnot, but really having little idea about the speed and trajectory in terms of long-term view.”

As humans, it’s clear we need to tackle the direness that is global warming, but the lion’s mane and its jelly comrades would really prefer that we didn’t. Not only do jellies grow faster in warmer waters, temperature is a pivotal factor in their reproduction. In some species, polyps will only develop as days grow longer in summer, but others instead wait until the water climbs to a certain temperature. Thus ever-hotter oceans in these times of global warming could make for more blooms.

Yeah, we didn't have to pay for this one, so hopefully the photographer is OK. Image: Wikimedia

In addition, global warming is monkeying with oxygen concentration in our seas, which is also great news for jellies. “Colder water holds more dissolved oxygen than warmer water,” said Gershwin. “So even a really slight warming—a degree, a half a degree, a quarter of a degree—we may not feel it, but it changes the amount of oxygen that the water can hold.”

And jellyfish are really good at living in oxygen-deprived water. Pretty much everything else in the sea? Not so much. “High-rate breathers,” such as beefy fish that need lots of oxygen to power their muscles, die off when jellyfish lazily cruise around, not the slightest bit fazed.

Then there’s the inflow of our sewage and fertilizers, nutrients that microscopic plants called phytoplankton go ga-ga for. Their populations explode, and are then eaten by their animal counterparts, zooplankton, which are in turn eaten by jellies. But when blooming phytoplankton die and decompose, the bacteria that feed on them suck still more oxygen out of the water.

Add all of this to the fact that we’re overfishing the hell out of our oceans—eliminating not just jellyfish predators but also their competition—and we have a gelatinous, stingy mess on our hands. “It’s probably really tempting to think about jellyfish as these evil beings, we should extinct them because they're bad,” said Gershwin. “But what they're doing, whether it’s stinging us or eating all of the fish eggs and larvae or clogging up power plants or whatever, they're just responding to what we're doing.”

So we just may have unwittingly assembled an ever-growing army of jellies, led by the outsized lion’s manes, for all-out assault. And this time there won’t be a boulder-wielding Sherlock Holmes to come to the rescue. Which is just as well if his associates insist on throwing dogs through windows.

Browse the full Absurd Creature of the Week archive here. Have an animal you want me to write about? Email matthew_simon@wired.com or ping me on Twitter at @mrMattSimon.