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Sacculina is a genus of barnacles that is a parasitic castrator of crabs. 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. Depending on the location, the prevalence of this unusual crustacean parasite in its crab host can be as high as 50%.[2]

Life cycle[edit]

The female Sacculina larva finds a crab and walks on it until it finds a joint. It then molts into a form called a kentrogon, which injects its 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 now 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 looks for a female Sacculina adult on the underside of a crab. He then implants himself into her body and starts fertilizing her eggs. The crab (male or female) then cares for the eggs as if they were its own, having been rendered infertile by the parasite.

When a female Sacculina is implanted in a male crab it will interfere 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.

Although all energy otherwise expended on reproduction is directed to the Sacculina, the crab develops a nurturing behavior typical of a 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 parasite eggs of the Sacculina are ready to emerge from the brood pouch of Sacculina, the crab performs a similar process. The crab shoots them out through pulses creating a large cloud of parasites. The crab then uses the familiar technique of stirring the water to aid in flow.[3]


More than 100 species of Sacculina are currently recognised:[4]


  1. ^ H. Boschma (1955). "The described species of the family Sacculinidae" (PDF). Zoologische Verhandelingen 27 (1): 1–76. 
  2. ^ Ross Piper (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press. ISBN 0-313-33922-8
  3. ^ Carl Zimmer (2000), Parasite Rex: Inside the Bizarre World of Nature's Most Dangerous Creatures, Free Press. ISBN 0-7432-0011-X
  4. ^ Geoff Boxshall (2012). "Sacculina Thompson, 1836". World Register of Marine Species. Retrieved October 21, 2012. 


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