Crabs get a bad rap. After all, people who are into cheerfulness call me “crabby” all the time. But let’s cut them some slack (the crabs, not the cheerful people—forget them), because a lot of these critters could be going through some stuff. And by stuff I mean what has to be one of the most disturbing and wildly complex parasitizations in the animal kingdom.

So the rhizocephalans are these species of barnacle, and unlike most barnacles, they aren’t content living life stuck to rocks. Indeed, they look nothing like barnacles. Well, they start out as regular barnacle-ish larvae, shaped like an oval, but that’s where the similarities end. Instead of developing into your classical shelled variety, they invade the bodies of various crab species. And not as mere tiny hitchhikers: After penetrating a crab’s shell, a rhizocephalan grows as meandering roots throughout its victim’s flesh, sometimes reaching nearly every part of its body.

And then things start getting weird.

Henrik Glenner

Like a Tree, Only It Grows Inside You and Instead of Drinking Water It Drinks Your Bodily Fluids

Life for a rhizo begins as an aforementioned larva, which is tasked with the seemingly impossible mission of not only finding the right species of crab (they tend to be able to infect just one type—that is, they’re highly “host-specific”), but somehow landing on it in the vastness of the sea. Rhizo babies appear in huge numbers, though, so by pure chance a few are bound to find their target. The rest—well, points for trying.

When one lands on a crab, it makes its way to one of the host’s many sensory hairs, known as setae, where the carapace is the weakest. Here the rhizo secretes a cement to anchor itself, just as a typical barnacle would. Next “it forms a so-called stylet, which is a hollow structure,” says Henrik Glenner of Norway’s University of Bergen. “It's almost arrow-shaped, and with this it penetrates the cuticle of the host.”

Henrik Glenner

With this syringe, the rhizo injects cells of itself into the crab’s hemolymph, the invertebrate equivalent of blood. And you know how bad a sinus infection feels? Well, the rhizo ends up in the hemolymph sinuses, much to the chagrin of the crab. It’s here where it starts to grow as roots, which have a covering that somehow protects them from a potential immune response as they work their way through the host. They’ll soak up massive amounts of nutrients from the crab, and the roots can end up pretty much anywhere, even in the claws.

All the while, the crab is still miraculously growing, periodically shedding its exoskeleton. But eventually it stops, likely because the rhizo is appropriating too much of its nutrients. It’s at this point that the parasite enters its next stage: sexy time.

The Merits of Being Pretty Much Just a Testicle

Because the crab is no longer molting its exoskeleton away, the rhizo can now extend itself out of the host, forming a mass on the crab's abdomen. “The funny thing about this is that this sac-like structure, the externa, is situated exactly where an adult female crab would have its egg mass,” says Glenner. “And we don’t know what the mechanism is, but the host considers the parasite as a part of itself.” The crab even takes care to groom and ventilate the sac, full of the rhizo's eggs. (If the rhizo has infected a male crab, the host will actually start transforming morphologically, widening its abdomen to more closely resemble a female. This serves as better protection for the sac: If it grows on a wider abdomen, it won’t overflow past the edge of the carapace.)

All the more incredible, the rhizo is pulling this all off without a brain of its own and only the remnants of a nervous system. And the commandeering doesn’t end at the crab caring for the parasite. Somehow, the rhizo directs the crab away from the general crustacean population into deeper waters, thus avoiding feeding competition with healthy crabs. Non-parasitized gravid (that is, preggers) females do this with them, since they’ll find better protection from their enemies in the depths.

Henrik Glenner

Now, it’s only female rhizos that infect crabs. Dwarf males look much like the larval form, and must somehow find not only a specific species of crab, but a specific species of crab that’s been invaded by a female. And that female has to be a virgin. The search, as you might expect, can be rather frustrating.

But should a male find a virgin female, he inserts himself into receptacles in her sac-like structure. “There the male changes form completely and becomes just a mass of cells,” says Glenner. “And actually it becomes a functional testicle, nothing more, and it's nursed by the female. Then they are united for life.” Again and again, the male produces sperm to fertilize her eggs.

The eggs will hatch right inside the female, and when she’s ready to release the larvae, she gets an assist from her host. Your regular unparasitized female crabs will raise themselves up and shake their abdomens to disperse their eggs, and so too do infected crabs shimmy as the rhizo pumps out its larvae, boosting them into the water column. And off they go to infect still more crabs.

'There the male changes form completely and becomes just a mass of cells. And actually it becomes a functional testicle, nothing more.' Henrik Glenner

As a final insult, during all of this the crab itself cannot reproduce, for the rhizo has sterilized it. A lot of parasites do this—energy that the host isn’t putting toward reproduction can instead go to the parasite. The rhizo may pull this off with some kind of chemical, or it may just be a matter of exhaustion for the crab. Remember that all this time the parasite has been sapping it of nutrients, so it could be that the crab has gone into dire survival mode, and reproduction is the first thing that goes.

“The crab is not a crab anymore,” says Glenner. “It's a device for the parasite because it cannot reproduce itself. But it behaves as if it were reproducing all the time.”

Yet it will not die. In fact, the parasite has a shorter lifespan than the crab and may therefore perish first, leaving the victim with the tell-tale scars of rhizo on its abdomen. That, and probably some serious existential angst.

Browse the full Absurd Creature of the Week archive here. Know of an animal you want me to write about? Are you a scientist studying a bizarre creature? Email matthew_simon@wired.com or ping me on Twitter at @mrMattSimon.