Creepy crab parasites: rhizocephalan barnacles

by Dr Marissa McNamara, Collection Manager of Crustaceans

If you think you’re having a bad day, be thankful you’re not a crab!

Sure, they have a great life (who wouldn’t want two big claws, or eyes on stalks?!) but crabs can become infected with some of the scariest parasites around: rhizocephalan barnacles.

Barnacles might not sound scary, or even particularly interesting, and most people probably don’t know what sort of creature they are. (If this includes you, don’t feel bad… barnacles were mistaken for molluscs until the 1830s.) For those readers with a thirst for knowledge, the animal underneath the plates is actually a crustacean. It doesn’t have proper eyes or a head, but it does have a mouth, a stomach, and feathery legs that it uses to feed. These legs are what give barnacles their scientific name: Cirripedia, meaning feather feet.

Most of the barnacles people encounter fall into two groups: acorn barnacles, which look like tiny volcanoes and attach to things like rock and jetties; and goose barnacles, which look like weird fleshy stalks and attach to floating debris.

Goose barnacle from Gold Coast, QLD. Image: Neville Coleman

So. Back to rhizocephalans.

While your average barnacle might not pose much of a danger to anything except a boat hull, rhizocephalans are anything but average. They don’t attach to rocks or floating debris. Instead they have one goal: find a host, and infiltrate.


When a rhizocephalan larva finds a crab she injects herself into it, and then begins to grow. She does not take any recognisable crustacean shape, but instead forms a creepy network of filaments known as an ‘interna.’ But she doesn’t stop there. Once her interna is formed she grows the other part of her ‘body’: the externa, which is her egg sac. This she forms under the crab’s abdomen, where the crab would normally carry her own eggs. But the crab will never carry her own eggs now, because she has been hijacked by the parasite. She won’t reproduce, and she stops moulting… she exists only as a zombie whose sole purpose is caring for the parasite’s eggs.

And what happens if this horrifying parasite infects a male crab, which is not equipped to carry eggs?

Simple! The rhizocephalan castrates the crab, effectively turning it into a female crab that will look after the parasite’s eggs.

You may be thinking of some nasty names to call these barnacles—and I wouldn’t blame you—but their scientific names are just as noteworthy. The most famous rhizocephalan is Sacculina carcini, which was described in 1836 from the European shore crab Carcinus maenas. While this barnacle is only found in Europe and Africa, don’t worry; there are over 200 rhizocephalan species worldwide, and we have our own horror right here in Moreton Bay: Sacculina granifera, which can be found on Blue Swimmer Crabs.

So the next time you think your day is bad, just remember… at least you don’t have an externa growing under your abdominal flap!

Sacculina granifera on Blue Swimmer Crab (Portunus armatus)

(And rest easy…if you consume a crab infected with Sacculina, the parasite will NOT grow inside you. Although that would make life a lot more interesting!)

Found something unusual?

Do you have an interesting question or mystery object? Our helpful and knowledgeable staff at the Discovery Centre can answer your questions through our Ask an Expert inquiry service.