The host crab is chemically castrated, and the distended growth at the base of the abdomen is the barnacle's brood chamber - and how does it breed?
"A male cyprid of the same species of barnacle attaches itself to the opening of this brood chamber, and then grows inside a special chamber within the female barnacle which itself is inside the body of the parasitised crab. This male growth forms into a testis. The female then effectively becomes a hermaphrodyte, which means that it can fertilise itself."
http://www.mesa.edu.au/friends/seashores/crab_parasitism.html
The host crab is chemically castrated, and the distended growth at the base of the abdomen is the barnacle's brood chamber - and how does it breed?
"A male cyprid of the same species of barnacle attaches itself to the opening of this brood chamber, and then grows inside a special chamber within the female barnacle which itself is inside the body of the parasitised crab. This male growth forms into a testis. The female then effectively becomes a hermaphrodyte, which means that it can fertilise itself."
A parasite of a parasite.