Most of us know barnacles as those small,
shelled creatures you find at the beach attached
to rocks and piers.
But, there’s one species of barnacle with
a very different sort of life cycle.
It’s a parasite that castrates crabs and
turns them into barnacle baby-making machines.
I’m Anna Rothschild, and this is Gross Science.
So this parasitic barnacle is called Loxothylacus
panopaei, and it starts out its life just
like most barnacles do—as a free-swimming
creature looking for a home.
And, home for this particular parasite is
inside of a mud crab.
When a young female barnacle finds a mud crab
(and let’s say it’s a male mud crab, though
they infect females as well) she undergoes
a transformation, losing her eyes and legs.
Then, she injects herself into the crab’s
circulatory system.
Once inside she starts forming a root system
throughout the crab’s body, castrating him
in the process.
Soon, a sac starts poking out from the crab’s
abdomen.
This is an “externa,” the reproductive
organs of the adult female barnacle.
The externa attracts male barnacles, and when
a male arrives he injects his own cells into
the sac, which grow into a testicle-like structure.
In fact, the adult male barnacle exists only
as a testicle, fertilizing the female barnacle’s
eggs.
You might be wondering what’s happened to
the crab after all this.
Well, believe it or not, he’s definitely
still alive, but he’s no longer quite himself.
To begin with, his abdomen has widened, so
he looks a little bit more like a female crab.
And, by the time the parasite’s eggs are
fertilized, this male crab is convinced that
he’s pregnant!
He cares for the eggs in the externa like
they’re his own babies, until the parasitic
spawn are released and the cycle begins again.
So, this is all pretty creepy, but why should
we care?
Well, these parasites can be extremely prolific—for
example, in some places in the Chesapeake
Bay up to 75% of the mud crabs are infected,
which means that none of those crabs can reproduce.
And, Loxothylacus panopaei isn’t the only
species of castrating, parasitic barnacle
out there.
There are other species that infect other
crustaceans, like blue crabs and shrimp, which
make up parts of our food supply.
So, while these barnacles can’t actually
infect humans, they’re not just a problem
for crabs—they could be 
a problem for us, too.
Ew.
