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.