A hostile takeover
Along the coasts of Europe, a bizarre biological heist unfolds. The perpetrator is Sacculina carcini, a species of barnacle that bears no resemblance to its rock-clinging relatives. Its target is an unsuspecting shore crab, often the common green crab (Carcinus maenas). The invasion begins when a free-swimming female cyprid larva, the barnacle's mobile stage, finds a suitable host. It attaches itself to a joint, often at the base of a bristle, and undergoes a radical transformation. Shedding its own shell, it injects a small mass of cells into the crab's body.
This mass of cells, now called the interna, migrates to the crab's gut. There, it grows a network of root-like tendrils that spread throughout the crab's entire body. These roots wrap around organs and nerve centers, siphoning nutrients directly from the host. The crab is now a living incubator. Its own biological functions are subordinated to the parasite's needs; growth ceases as the crab is no longer able to molt. All of its energy is redirected to feeding its internal occupant.
Mind control and reproduction
Once the interna is established, it begins to manipulate the crab's hormonal system. The parasite effectively castrates its host, destroying the crab's gonads. A portion of the parasite then erupts from the crab's abdomen, forming an external sac called the externa. This sac is positioned precisely where a female crab would normally brood her own eggs.
The manipulation is comprehensive. If the host is a male, the parasite feminizes him. His abdomen widens and flattens to resemble a female's, and his claws may shrink. He even begins to perform female grooming behaviors and mating dances. Both male and female hosts are compelled to care for the externa as if it were their own clutch of eggs, cleaning it and protecting it from harm.
To complete its life cycle, a tiny free-swimming male cyprid finds the externa and injects its own cells inside, which then fertilize the female's eggs. When the parasite's larvae are ready to hatch, the crab is compelled to find a high rock, where it bobs and wafts the water to release the new generation of parasites. The host crab can live for one to two years after infection, serving as a permanent vehicle for the barnacle's reproduction. In some crab populations, the prevalence of Sacculina carcini can be as high as 50 percent.