The rhizocephala are a superorder within the subclass Thecostraca, the group of crustaceans that includes all the barnacles, but their form is highly modified from the familiar acorn and gooseneck barnacles. The only visual indication we have that these creatures are related to the other barnacles is their nauplius and cyprid larvae. In the normal barnacles, these larvae find a rock, stick themselves to it, grow a shell, and spend the rest of their days filter feeding.
Rhizocephalan larvae do not settle on rocks. Instead, the females settle on other crustaceans–usually crabs, and usually on the gills, where the crab’s exoskeleton is thinnest. This is important, because soon after settling, the larva injects its soft innards into the crab. This may take place through one of the larva’s antennae, wielded like a hypodermic needle, or the larva may molt and stick itself on the crab while it develops a special stylet inside its body. When the time is right, it everts this poker into the crab and injects its guts as a worm-like body called a vermigon.
This is where it starts to get really awful. The injected mass of visceral cells, now called the “interna,” starts to grow inside the host’s fluid-filled cavities, branching out root-like through the entire body, but concentrating on the digestive tract. Somehow, the parasite keeps the host’s immune system from attacking this network of invasive threads while it grows, drawing nourishment continuously from the host’s precious bodily fluids.
When mature, the female interna extrudes an appendage out of the host’s body. This is called the externa, and it is little more than a gently-pulsing sac of gonads, attached to the host’s body by a short stalk, emitting a trail of pheremones to attract male larvae. When these larvae arrive, they land on the externa, and inject their soft insides into it. They then migrate into special seminal receptacles and fuse with the female, becoming nothing more than soft clumps of sperm-producing tissue, themselves parasitic on the already-parasitic female. Eggs are fertilized and released, and the cycle begins again.
But wait–it gets even sicker. In many cases, the externa is extruded under the crab’s tail flap, where it would carry its own eggs. Some species of rhizocephalans actually chemically castrate the crab, and then modify its behavior–that is, perform mind control–compelling it to take care of the parasite’s gonads as if they were the crab’s own children. This works even in males, who don’t ever carry eggs, and never exhibit any of these behaviors on their own. In these cases, the crab stops growing, and never molts again. It spends the rest of its days as a kind of zombie, eating food that will be sucked out of its digestive tract by the parasite that destroyed and replaced its reproductive system.
Now aren’t you glad you know all that?
Why do things like this even exist? Don't you think it'd be too evolutionarily complicated?
