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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.
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.
