Barnacles lead mundane lives. They spend their time attached to old piers and sunken ships, waving their tiny arms about all day to grab edible morsels from the water. But one barnacle decided that arm-waving just wasn’t for it and it turned down a more sinister route. It’s the body-snatching barnacle known as Sacculina.
As a larva, Sacculina is tiny, translucent and free-swimming, but when a female finds and latches onto a shore crab, she transforms. She finds a chink in the crab’s armour – usually a leg joint – and jabs a long hollow dagger into it. She then literally injects her soft parts into the crab’s insides, leaving behind a husk of what was once the rest of her body. It’s a very extreme version of moulting. Once inside the crab, Sacculina branches out throughout much of its host like the roots of a plant, absorbing nutrients directly from the crab’s bloodstream.
Now the female Sacculina turns its attention towards reproduction. She forms an external brood sac, which resembles the egg mass of the female crab she is residing within. Then, things get even stranger. A male Sacculina arrives and injects himself into the crab’s body just like the female did, but these injected cells become a spine-covered larva that migrate to a special chamber inside the female Sacculina before transforming once again, this time into a pair of testes. With the male now in place, the female Sacculina produces its brood chamber, which protrudes from beneath the crab’s abdomen. You can see the parasite in the picture below as a yellowish mass underneath the crab.
This is all bad news for the crab. The parasite absorbs so much nourishment from its host that there is not enough room for the crab’s sex organs to mature properly and the host becomes sterile. Interestingly, though, the crab behaves as though Sacculina’s egg mass is its own, protecting and cleaning it and even maintaining a flow of water around it to provide the developing parasitic larvae with sufficient oxygen. Undoubtedly Sacculina is responsible for this unusual behaviour, somehow producing substances that control or dupe the crab.
But what if the infected crab was a male? Well, that’s no problem – Sacculina simply interferes with his hormones so that the male not only changes his body shape to that of a female, complete with a wider and flatter abdomen, but he also starts to behave like one as well. He’ll even start performing the mating dance of the female crab.
The crab, whether female or emasculated male, now lives only to serve its parasitic master. It stops growing and moulting. It no longer regrows claws that it may lose. It doesn’t even try and mate with other crabs. All of those things would funnel energy away from Sacculina. The crab instead spends the rest of its life raising the offspring of its cancerous parasite.