Particularly if you're a crab. As Kevin Zelnio at Deep Sea News explains, "Imagine you have a beach ball attached to your . . ." Um, yeah. Rhizocephala are curious creatures. They are actually in the Cirripedia, the group containing your friendly neighborhood penis-waving barnacle. They look nothing like a barnacle (in case you hadn't noticed). We know they are a barnacle because they share the same larval stages and characteristics only found in the barnacles: cyprid larvae. . . . This poor anomuran crab, Paralomis hirtella, has the misfortune of getting her entire reproductive system hijacked by some little punk rhizocephalan. . . . The pink sack is the brood chamber (called an externa), where all the little eggs are being produced and plot to take over the world. Only females make the externa. Rhizocephala means "root head". You can see the "roots" just underneath the externa where I broke off the tail of the crab (accidentally, I swear!). The roots penetrate throughout the host and tap into its body to get the energy it needs to reproduce. Since the rhizocephalan hijacks the reproductive system of its host, the crab is tricked into caring for the externa as if it were its own brood. This is even the case in male crabs. Since crabs moult after they release their eggs, they are also tricked into not moulting. Ever Again. If they did moult then they might shed off part of the rhizocephalan. The crab also never reproduces. Ever Again.
Read more about it -- and see more of Kevin's photos -- at the link.