Species: Members of the genus Sacculina
Range: The underside of unfortunate crabs pretty much everywhere
Weird Feature(s): Crustacean castration, body snatching
Most parasites are content with slowly killing you. Others have the decency to let you live your compromised life while they mooch off your bodily fluids. But species within the genus Sacculina want more than your blood–they want your brain.
Sacculina species belong to the same class as barnacles (Maxillopoda). And much like other barnacles, their larval stage looks nothing like their adult stage. In their larval stage, female Sacculina carini are small free-swimming critters that hang out in the water column until they find a crab to hang onto. Once a larvae has found a crab, it crawls along the crab’s shell until it finds an easy-ish point of entry into the crab’s soft body–usually one of the crab’s joints or the base of one of its hairs.
Once its found this easy entry point, Sacculina carcini sheds its outer shell and squeezes its soft body under the crab’s carapace. Once inside, it grows a network of tendrils which extend throughout the digestive system, leeching nutrients from the crab. The tendrils also extend into the host’s nerve centers including the cerebral ganglion–the closest thing they have to a brain. It’s at about this time that Sacculina carcini sterilizes the host for the next round of horror.
Soon, the parasite forms a massive reproductive unit that hangs off the bottom of the crab and can pump out hundreds of eggs a day. Since the sack is in the same place that the crab’s eggs would be, it’s only fitting that Sacculina alters the behavior of the crab to actually care for the eggs as if they were its own.
Male Sacculina (who never grow past their swimming larval stage) soon start paying visits to these egg blobs and fertilize the eggs inside. Once the new larvae are ready to leave the sack, the female Sacculina triggers the crab to enact a different stage of parenting by climbing to the top of a high rock and actively dispersing them, just as she would with her own larvae.
Judging from Sacculina‘s MO, you might think that being a male crab means you’re safe. Unfortunately, Sacculina is not gender biased. If it happens to crawl inside a male instead of a female crab, it’s satisfied to simply castrate the male crab and release hormones into its blood stream that make it act like a lady. Yes, you read that right. Sacculina effectively changes the gender of its male hosts–in both form and function–inducing the same nannying effect it’s able to force out of the females.