Sacculina is a genus of barnacles that is a parasitic castrator of crabs. They belong to a group called Rhizocephala. The adults bear no resemblance to the barnacles that cover ships and piers; they are recognised as barnacles because their larval forms are like other members of the barnacle class Cirripedia. The prevalence of this crustacean parasite in its crab host can be as high as 50%.[2][3]
Sacculina live in a marine environment. During their larval stage they are pelagic, but as they form into adults they live as ectoparasites on crabs. Their primary host is the green crab, which is native to the Eastern Atlantic Ocean. Though these crabs have spread to other bodies of waters, it is not believed that Sacculina barnacles have traveled with them to these new localities.[4]
The body of the adult parasite can be divided into two parts: one part is called the "externa" where the bulbous reproductive organ of the parasite sticks out of the abdomen of the host. The other part is called the "interna" which is inside the host's body. This part is composed of root-like tendrils that wrap themselves around the host's organs, which gives its group name of Rhizocephala, meaning "root-head". Through microCT scans, these roots have been discovered to wrap around certain organs of the body, with most around the hepatopancreas of crustaceans. This area is primarily for absorbing nutrients, which would explain why most concentrate in that region. In a similar species called Briarosaccusroots were seen extending to the brain and central nervous system, which is a hypothesis to help explain how parasites like these can manipulate their hosts' behavior.
The female Sacculina larva finds a crab and walks on it until she finds a joint. She then molts into a form called a kentrogon, which then injects her soft body into the crab while her shell falls off. The Sacculina grows in the crab, emerging as a sac, known as an externa, on the underside of the crab's rear thorax, where the crab's eggs would be incubated. Parasitic Sacculina destroy a crab's genitalia, rendering the crab permanently infertile.
After this invasion of the Sacculina, the crab is unable to perform the normal function of molting. This results in a loss of nutrition for the crab, and impairs its overall growth. The natural ability of regrowing a severed claw that is commonly used for defense purposes is therefore lost after the infestation of Sacculina.
The male Sacculina 'larva' looks for a female Sacculina on the underside of a crab. He then implants his cells into a pocket in the female's body called the "testis", where the male cells then produce spermatozoa to fertilize eggs.