Savagery Among the Seagrass
Rhizocephalan Sacculina carcini erupting from the belly of its green shore crab host.
Galindo L.-A. (CC BY 4.0)
If you've ever walked along a rocky shoreline, taken a trip to a pier or gotten up close and personal with a whale, you've probably seen the off-white speckle of barnacles. But what you might not know is that those shoreline barnacles are the white sheep of a devilish family.
Nauplius and cyprid larvae of Balanus.
Anonymous - 1911 Encyclopædia Britannica, vol. 7, Crustacea article (Public Domain)
Barnacles are crustaceans, distant relatives of crabs, lobsters and shrimp. And while they may look like clams or corals, one look at their young will reveal their crustacean affinities. Barnacles hatch out of their eggs as nauplius larvae—just a head with three pairs of swimming legs. Many other crustaceans such as shrimp and krill also start out life as these little swimming heads, though their paths diverge after that. Whereas a shrimp nauplius will grow its tail and claws, a barnacle nauplius will instead change into another type of larva called a cyprid. The cyprid larva has no mouth and cannot eat, instead relying on its miniscule fat reserves to keep it alive. With its limited time it must find a suitable place to attach, be it a rock, a ship or a whale. Once it finds a substrate it's satisfied with, it will glue its head with a powerful organic cement and change its form one final time.
The adult barnacle is essentially a little shrimp stuck headfirst to its chosen substrate, encased in an armoured house and catching drifting plankton by kicking its feathery legs. Some may cement themselves directly to their substrate, while others may grow a muscular stalk on their heads to raise themselves off their substrate and move around a little.
And once upon a time, this was how all barnacles lived. But nature abhors a vacuum, and so the barnacles' habit of attaching to other animals eventually turned sinister.
The majority of barnacles are actually parasites of some kind or another. Rhizolepas and Anelasma hail from the ranks of the classic shore barnacles but root their stalks into the bodies of worms and sharks respectively, while coral-burrowing barnacles chew up the limestone bodies of their unwilling hosts. But all of these are chump change compared to the real master freeloaders of the barnacle family: the Rhizocephala ('root-heads').
Horrifying image of Anelasma barnacles attached to a shark host.
Charles L. Griffiths (CC BY-SA 4.0)
To experience the true horror of the Rhizocephala, imagine for a minute that you are a male green shore crab (Carcinus maenas). You live your life dodging predators and finding food among the seagrass meadows of the New England coast and you've done pretty well for yourself, making it to adulthood while so many of your siblings fell prey to octopuses, seals and seagulls. Its a nice warm spring day and you're looking forward to next year, when you'll be mature enough to breed for the first time. But unknown to you, a female rhizocephalan cyprid of the species Sacculina carcini has caught your scent. Her legs beat furiously, propelling her tiny body towards you. You are completely unaware as she lands on your shell, preparing to begin her grim work.
The S. carcini cyprid does not differ much from her free-living cousins. Like them, she has no mouth and must find her chosen substrate within the few days she can live off her internal fat supplies. Unlike them, her chosen substrate is not a rock or a whale but rather your living flesh. She scurries along to one of the many joints in your exoskeleton where she uses a hypodermic needle attached to her head to stab through your joint membrane. Once your skin is breached she injects a small cluster of cells, the vermigon, into your bloodstream. The rest of her, including her brain, her muscles and her shell, falls away dead. You feel nothing, for she is much too small to set off your body's defences.
In the months that follow, you continue to live like normal, unaware of the growing horror inside your body. The S. carcini vermigon grows and grows, transforming from a single thread of cells into a branching network of 'roots' which wind their way around your intestines and siphon nutrients from your blood. But in the winter months, before the mating season, something happens. When you molt your shell, your tail is wider. The swimming legs on your abdomen are nowhere to be seen. You feel... weirdly maternal. And in the spring, when breeding season starts, something truly horrible happens.
Sylon hippolytes doing the body-snatching routine to a shrimp.
Christoph Noever/Universitet i Bergen (CC BY 4.0)
It begins with a strange pressure at the base of your tail, which turns into a splitting sensation as a mass of flesh erupts from between the plates of your exoskeleton, right where your genitals should be. But instead of trying to excise this strange tumour from your body, you instead start to fan and clean it, just like a female crab would do with her eggs. For the roots of S. carcini have not only invaded your intestines—they have also infiltrated your brain. You are fully convinced you are a female crab, and that the tumourous mass on your genitals is a freshly-fertilised cluster of eggs. Indeed, there is some truth to this delusion—the parasite's feminizing hormones are so powerful that you would remain a female even if it was surgically excised from your body. But your gonads are shriveled and barren, for S. carcini is in charge of your body now. Her externa, the fleshy mass bursting from your body, emits a powerful pheromone into the water that attract the attention of male S. carcini cyprids. They land on the externa and use their needles to inject their vermigons inside, where they grow into nothing but masses of sperm-producing stem cells: the simplest adult males of any species in the animal kingdom. With her eggs fertilised by her microscopic suitors, the female S. carcini gets down to producing millions of eggs, kept clean and healthy all the while by her hypnotised host. When the externa is mature and ready to release its eggs, she compels you to climb to the tallest rock you can find, where you fan your tail to disperse her eggs into the current. You are fully convinced that you have raised your own young, even as S. carcini's children prepare to seek out their own hosts to bodysnatch. You will continue to dance to her tune for as long as she lives, which very well may be the rest of your own natural lifespan.
And S. carcini is not alone. All sorts of crustaceans, from crabs to lobsters to shrimp to even other barnacles can also experience the horror of having a giant bug-tumour erupt from their nether regions. Though we don't know much about these species, micro-CT scans of their roots indicate that many of them practice the mind-controlling habits of Sacculina carcini. They are so successful and so effective that in many areas all hosts will eventually fall victim and have evolved to race to the reproductive finish line before they are chemically castrated by a bodysnatching barnacle. Sacculina carcini in particular is so good at severing bloodlines that it has been considered as a biological control for invasive green shore crab populations wreaking havoc in places like New Zealand.
The cleverness and sadism of the Rhizocephala far exceeds that of any mosquito or leech the continents have to offer, reminding us that we land animals have it easy. But we shouldn't get too complacent—who knows what terrifying and wonderful things evolution will come up with in the eons to come?
