Sacculina carcini

Sacculina carcini, the crab hacker barnacle, is a species of barnacle in the family Sacculinidae that is a parasite, in particular a parasitic castrator, of a crab. The crab that most often is used as a host is the green crab, the natural range of which is the coasts of Western Europe and North Africa. S. carcini is a parasite of a crab, most commonly the green crab (Carcinus maenas), and the parasite's range largely coincides with that of this host, the coasts of Western Europe and North Africa. However, the green crab has been expanding its range and has become established in both North and South America, Southern Africa and Australia, so it is possible that the parasite is now present outside its natural range. Other crabs in the swimming crab family, Portunidae, that sometimes act as hosts in Western Europe are the flying crab (Liocarcinus holsatus), the harbour crab (Liocarcinus depurator) and the velvet crab (Necora puber). These crabs all live in shallow water over sandy or muddy substrates. A female Sacculina carcini larva settles on a suitable crab host and crawls across its surface until it finds a suitable spot such as the base of a seta (bristle). It then develops into a form called a kentrogon, which inserts a stylet into the crab and pushes its way inside. From there it moves through the inside of the crab, in due course pushing out a sac, known as an externa, on the underside of the crab's abdomen. The part remaining inside, the interna, develops tendrils which spread throughout the crab; they absorb nourishment and enable the parasite to control the behaviour of its host. The presence of the parasite retards the development of the crab's gonads, which eventually atrophy. The parasite causes a male crab to develop certain feminine characteristics including the broadening of its abdomen, while in females, the abdomen becomes narrower and the pleopods degenerate. The eggs of the parasite develop in the externa and both male and female crabs carry these eggs around, secured under their abdomen, in the way that female crabs normally care for their own brood (but males never do). If the parasite is experimentally removed from the host, female crabs will usually regenerate their ovaries, but in males, sex change takes place and they develop ovarian tissue. The eggs inside the externa are fertilised by male larvae which enter the sac through a pore. These males are tiny, never become adults and soon die. However, the female, including the externa, can live for as long as the crab host survives, perhaps one or two years. Multiple eggs are produced every day and remain in the sac for about six weeks. The behaviour of the host is manipulated to the extent that it no longer grows, moults or regenerates any limbs that are lost, and when the parasite eggs are ready for release, it will climb onto a rock, bob about to release them and waft them on their way.