Species of the week: the parasitic barnacle Sacculina

Krystal Deutsch describes a parasite which chemically castrates its host

Sacculina is a parasitic barnacle that depends on crabs for its growth and reproduction. A female Sacculina larva finds a crab, sheds its hard outer layer and injects her soft inner body through a joint in the crab’s shell, entering the crustacean’s body and using its energy.

The Sacculina larva grows inside the crab and develops a sac which hangs outside the crab’s body where the crab’s eggs would normally incubate. Sacculina renders the crab infertile – the parasite wants the energy that the crab would invest in reproduction to be conserved for its own growth and life cycle. Infected crabs are also prevented from moulting their shells and re-growing lost limbs, further conserving energy for the Sacculina.

In male crabs the parasite makes some remarkable changes to the crab’s body and behaviour. Sacculina releases hormones that chemically castrate the male crab, change the crab’s body to resemble a female of the species and even make the crab execute female mating dances.

Male Sacculina find an infected crab and fertilize the eggs in the female’s sac dangling from the crab’s thorax. During reproduction in healthy crabs, the female finds a high rock and releases fertilized eggs from its brooding sac. Parasitized crabs perform the same behaviour, but inadvertently release a cloud of Sacculina eggs. The crab nurtures the Sacculina eggs as if they were its own offspring, and once the larvae hatch from the eggs and are released into the sea the process begins once again.

Read more about parasites from Krystal.