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Sacculina carcini


broken white to light brown


nutrients, dissolved in the blood of the crab



  • Dut: Krabbenzakje
  • Lat: Sacculina carcini
  • Eng: see Latin
  • Ger: Parasitischer Wurzelkrebs
Sacculina carcini, NIOZ (

Sacculina carcini

Sacculina carcini is a parasite related to goose barnacles and common barnacles. The animal looks like an irregularly shaped knob under the shield covering a crab's belly. In the Netherlands, Sacculina carcini is mostly found in shore crabs and flying crabs. With its diffuse branching structure, the parasite spreads through the body of the crab. Infected crabs do not die but can no longer reproduce of shed their shell. The crab eventually weakens and dies, as Sacculina carcini sucks up all the food from the host's vascular structure.

  • Distribution and habitat

    Sacculina carcini is found anywhere where crab species live, which it uses for food and housing. In the Netherlands, you find it wherever you find shore crabs and the flying crabs.

  • As aliens

    The life of a Sacculina carcini would make a good alien horror film. It begins life as a swimming larva. As soon as the female larva smells a crab, she approaches it and looks for a soft spot in one of its joints. Via this soft spot, she spouts several cells into the body, which form a slug. She discards the majority of her body in the process (a kind of molting). This slug crawls deep down in the underside of the crab, where it starts to grow. It develops multiple tendrils which start to move throughout the crab's body, sometimes even surrounding the eyes.

    A knob starts to form (see photo) in the underside, situated precisely where the crab would normally carry its eggs. The crab doesn't realize that the knob is a parasite, mistaking it for her eggs. Therefore, she takes care of the knob with the same amount of love she would give to her eggs, which is exactly what the parasite wants.

    Up till now, the female parasite inside the crab has a comfortable life. However, she needs to find a partner and lay eggs. When a male Sacculina finds a female sitting in a desirable spot in a crab, then it too will enter the female Sacculina via a special opening. He remains inside the female the rest of his life, while the female remains inside the crab. He regularly fertilizes the egg  cells so that new larvae are born every week. In that way, an entire crab population can be infected with this parasite.

  • Confused crabs

    Sacculina not only changes the crab's body, it also changes its habits. Contaminated crabs are no longer able to molt so that lost claws or legs cannot rejuvenate. In addition, such crabs are constantly preoccupied with eating. Sacculina lives from the nutrients ingested by the crab, so the host has to eat all the time to produce sufficient energy for the two of them.

    Female crabs care for the sack as if they were her own. They groom the eggs by scrapings off algae and fungi and help them out of the pouch when they hatch. They even wave their claws to create more current to carry them away.

    Male crabs contaminated with Sacculina start acting even more unnatural. Their body takes on a female shape so that the Sacculina will fit better on the underside. They form female habits by caring for the eggs, while males normally have no eggs whatsoever.