Sacculina
A barnacle that dissolves itself into a crab and puppeteers it from inside
No photo available for Sacculina
Superpower
A free-swimming female Sacculina larva finds a joint in a crab's shell, sheds her entire external body like an empty syringe, and injects only her soft internal cells into the crab. She then grows thread-like roots throughout the crab's body — into every organ, around the nerve ganglia, along the eyestalks. The crab is now controlled. It stops molting, stops reproducing, and begins tending the external Sacculina egg sac on its underside as if it were the crab's own eggs — including performing the rocking, aerating dance that female crabs use for their own clutches.
Overview
Sacculina is a barnacle — completely unrecognizable as one after infection, but genetically confirmed as a member of Cirripedia. It parasitizes shore crabs and swimming crabs worldwide. Once the roots are established, the crab's energy is redirected entirely toward the parasite's nutrition and reproduction. Male crabs are chemically feminized — their behavior changes to match a brooding female's. The lump under the crab's abdomen that people sometimes mistake for a diseased egg mass is the Sacculina's reproductive body. The crab lives, feeds, and defends the parasite's offspring as its own.
Found in
Coastal waters globally wherever shore crabs (Carcinus maenas) and related species live, from Europe and the Americas to Asia. Infection rates in some harbors exceed 40% of the local crab population.
Things worth knowing
- 1
If a second male Sacculina larva arrives at an already-infected crab, he doesn't attempt to infect the crab himself — he finds the external Sacculina body and injects himself into it, becoming her permanent internal reproductive partner.
- 2
The infected crab never molts again — the parasite suppresses the entire molt cycle. The crab's shell gradually becomes worn and pitted but is never shed.
- 3
After infection, male crabs develop the hip-swaying aeration dance only uninfected female crabs perform — a behavior controlled entirely by the parasite overriding the crab's nervous system.
- 4
Sacculina infections appear to suppress the immune response of the host so completely that researchers studying crustacean immunity use infected crabs as a model for studying immune suppression.
- 5
When the host crab eventually dies, the Sacculina dies with it — the two are permanently bound together from the moment of infection.