Ray Lankester had nothing but contempt for Sacculina, the barnacle that degenerates practically into a plant. He was appalled by the way it had clambered down the ladder of evolution, a symbol of all things backward and lazy. Strange, then, that Sacculina now turns out to be an emblem for just how sophisticated a parasite can get.
Lankester’s mistake didn’t stem simply from a loathing for all parasites; biologists of his day just didn’t know much about Sacculina. It’s true that these parasites start life as free-swimming larvae. Through a microscope they look like teardrops equipped with fluttering legs and a pair of dark eyespots. Biologists in Lankester’s day thought Sacculina was a hermaphrodite, but in fact, it comes in two sexes. The female larva is the first to colonize a crab. She has sense organs on her legs that can catch the scent of a host, and she will dance through the water until she lands on its armor. She crawls along an arm as the crab twitches in irritation or perhaps the crustacean equivalent of panic. She comes to a joint on the arm, where the hard exoskeleton bends at a soft chink. There she looks for the small hairs that sprout out of the crab’s arm, each anchored in its own hole. She jabs a long hollow dagger through one of the holes, and through it she squirts a blob made up of a few cells. The injection, which takes only a few seconds, is a variation on the moulting that crustaceans and insects go through in order to grow. A cicada sitting on a tree separates a thin outer husk from the rest of its body, and then pushes its way out of the shell. It emerges with a new exoskeleton that stays soft long enough to stretch as the insect goes through a growth spurt. In the case of the female Sacculina, however, most of her body becomes the husk that is left behind. The part that lives on looks less like a barnacle than a microscopic slug.
The slug (whose existence was discovered only in 1995) plunges into the depth of the crab. In time it settles in the crab’s underside and grows, forming a bulge in its shell and sprouting the roots that so appalled Lankester. Biologists still call these things roots, but they are hardly like what you find under a tree. Fine fleshy fingers cover them, much like the ones lining our own intestines or the skin of a tapeworm. Unlike the exoskeleton of a regular crustacean, it is never moulted. Instead, the roots draw in nutrients dissolved in the crab’s blood. The crab stays alive during this entire time; you can’t tell it apart from healthy crabs as it wanders through the surf, eating clams and mussels. Its immune system can’t fight off Sacculina, and yet it can go on with its life with the parasite filling its entire body, the roots even wrapping around its eyestalks.
The female Sacculina’s bulge grows into a knob. Its outer layer chips away, slowly revealing a portal at the top. She will remain at this stage for the rest of her life unless a male larva finds her. He lands on the crab and walks along its body until he reaches the knob. At its summit, he finds the pin-sized opening. It’s too small for him to fit into, and so, like the female before him, he moults off most of himself, injecting a vestige of it into the hole. This male cargo—a spiny, reddish brown torpedo a hundred-thousandth of an inch long—slips into a pulsing, throbbing canal, which carries him deep into the female’s body. He casts off his spiny coat as he goes, and in ten hours he ends up at the bottom of the canal. There he fuses to the female and begins making sperm. There are two of these wells in each female Sacculina, and she typically carries two males with her for her entire life. They endlessly fertilize her eggs, and every few weeks she produces thousands of new Sacculina larvae.
The crab begins to change into a new sort of creature, one that exists to serve the parasite. It can no longer do the things that would get in the way of Sacculina’s growth. It stops moulting and growing, which would funnel away energy from the parasite. Crabs can typically escape from predators by severing a claw and regrowing it later on. Crabs carrying Sacculina can lose a claw, but they can’t grow a new one in its place. And while other crabs mate and produce a new generation, parasitized crabs simply go on eating and eating. They have been spayed. The parasite is responsible for all these changes.
Despite being castrated, the crab doesn’t lose its urge to nurture. It simply directs its affection toward the parasite. A healthy female crab carries her fertilized eggs in a brood pouch on her underside, and as her eggs mature she carefully grooms the pouch, scraping away algae and fungi. When the crab larvae hatch and need to escape, their mother finds a high rock on which to stand, and she bobs up and down to release them from the pouch into the ocean current, waving her claws to stir up more flow. The knob that Sacculina forms on a crab sits exactly where the brood pouch would be, and the crab treats the parasite knob as if it were its own pouch. She strokes it clean as the larvae grow, and when they are ready to emerge, she forces them out in pulses, shooting out heavy clouds of parasites. As they come spraying from her body she waves her claws to help them on their way. Male crabs aren’t out of reach from Sacculina’s powers, either. Males normally develop a narrow abdomen, but infected males grow abdomens as wide as females, wide enough to accommodate a brood pouch or a Sacculina knob. A male crab even acts as if he has the female’s brood pouch, grooming it as the parasite larvae grow and bobbing in the waves to release them.
Simply living within another organism—locating it, traveling through it, finding food and a mate inside, altering the cells that surround it, outwitting its defenses—is a tremendous evolutionary accomplishment. But parasites such as Sacculina do more: they control their hosts, becoming in effect their new brain, and turning them into new creatures. It is as if the host itself is simply a puppet, and the parasite is the hand inside.
This puppetry takes different forms depending on the particular parasite and what it needs from its host at its particular stage of life. When a parasite has first settled into a comfortable spot in its host, food is the first order of business. Once a tobacco hornworm has been rendered defenseless by the viruses of the parasitic wasp Cotesia congregata, the wasp’s eggs are ready to hatch and grow. Rather than just passively soak up the food around it, the wasp changes the way its host eats and digests its food. The more wasps in a given host, the bigger the host will grow—up to twice its normal size. And once the caterpillar eats a leaf, the wasps alter the way it breaks it down. Normally a hornworm would convert a lot of the leaf into fat, a stable form of energy that it can store away for the time when it will fast inside its cocoon. But once it is infected by wasps, the hornworm turns its food into sugar, a quick source of energy that the parasites use for fast growth.
A parasite lives in a delicate competition with its host for the host’s own flesh and blood. Any energy that the host uses itself could go instead to the growing parasite. Yet, a parasite would be foolish to cut off the energy to a vital organ like the brain, since the host would no longer be able to find any food at all. So the parasite cuts off the less essential things. As Cotesia congregata robs the caterpillar of its fat stores it also shuts down its host’s sex organs. Male caterpillars are born with big testes, and normally they channel a lot of the energy from their food into building them up even more. When a parasitic wasp lives inside the male, however, the testes shrivel up. Castration is a strategy that any number of parasites have hit on independently—Sacculina does it to crabs, and blood flukes do it to the snails they invade. Unable to waste energy on building eggs or testes, on finding a mate, or on raising young, a host becomes, genetically speaking, a zombie: one of the undead serving a master.