Sacculina

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%.
Habitat.
"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.
Anatomy.
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 dendrils 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 "Briarosaccus" roots were seen extending to the brain and central nervous system, which could help explain how parasites like these can manipulate their hosts' behavior.
Life cycle.
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 injects her soft body into the crab while its 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.
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 its cells into a pocket in the female's body called the "testis", where male cells produce spermatozoa to fertilize eggs. Parasitic "Sacculina" destroy a crab's gonads, rendering the crab permanently infertile. 
When a female "Sacculina" is implanted in a male crab, it interferes with the crab's hormonal balance. This sterilizes it and changes the bodily layout of the crab to resemble that of a female crab by widening and flattening its abdomen, among other things. The female "Sacculina" then forces the crab's body to release hormones, causing it to act like a female crab, even to the point of performing female mating dances. If the parasite is removed from the host, female crabs will normally regenerate new ovarian tissue, while males usually develops complete or partial ovaries instead of testes.
Although all energy otherwise expended on reproduction is directed to the "Sacculina", the crab develops a nurturing behavior typical of a normal female crab. The natural hatching process of a crab consists of the female finding a high rock and grooming its brood pouch on its abdomen and releasing the fertilized eggs in the water through a bobbing motion. The female crab stirs the water with her claw to aid the flow of the water. When the hatching larvae of "Sacculina" are ready to emerge from the brood pouch of female "Sacculina", the crab performs a similar process. The crab shoots them out in pulses, creating a large cloud of "Sacculina" larvae. The crab uses the familiar technique of stirring the water to aid in flow.
Life span.
"Sacculina" are primarily host dependent so their life span matches that of their hosts. Crabs usually have a life span anywhere from 1 to 2 years.
Biological control agents.
"Sacculina" has been suggested to be used as a type of biological control agent to help reduce the populations of the invasive green crab. This is controversial because "Sacculina" can also use native crab species as their host.
Species.
More than 100 species of "Sacculina" are currently recognised: