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The-Episiarch's avatar

Rhizocephalan larva infecting a crab

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Rhizocephalans are a group of parasitic barnacle which infects decapod crustaceans (such as crabs and shrimps). In the process it sends root-like tendrils throughout the body (the name rhizocephalan means "Root Head"), castrates the crab, and take control of its little mind, forcing the crab to take care of the barnacle's own brood as if they were the crab's own young. The most famous species out of that group is Sacculina carcini ([link])

But before they get to that stage, they start out much like other barnacles as a little free-swimming larva call a cypris. The cypris is tiny creature, less than 0.2 mm in length. Whereas the cypris of most barnacle would settle on a rocky platform (or the hull of a ship) then start the process of moulting and growing into a filter-feeding adult, the cypris of rhizocephalans settle on the carapace of a decapod, such as a crab.

The cypris settle at the base of one of the hair (call seta) on the crab's shell where the exoskeleton is at its thinnest. It then secretes a substance to cement itself in place - much like other barnacles. But unlike ordinary barnacle, when the rhizocephalan cypris undergoes its first moult (within minutes of settling on the host), it forms a structure call a kentrogon which is in effect a living hypodermic needle.

The kentrogon pierces the thin exokeleton at the base of the seta using a structure call a stylet and squirts the parasite (which at this stage is a tiny blob call a Vermigon) into the crab's body, where its will quickly grow in the crab's blood (unlike us, crustaceans and insects don't have blood vessels, instead their organs and muscles are bathed in a blood-like fluid call hemocoel sloshing around inside their exokeleton), eventually take over the crab's entire body (like so: [link]).

**NOTE: After :iconbanvivirie:'s suggestion (see below), I am submitting this to the context being held at BioIllustration [link] based on Prompt 2 (Create an illustration that includes a microscopic close-up of some part of the organism).**
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EmmetEarwax's avatar
Clearer explanation than wiki gave me.
I wonder why sacculina and other species deviated so wildly from normal barnacles.
(endamoeba coli is an usually commensal in our gut, but endamoeba histolytica strip-mines the intestinal lining, causing  no end of havoc. Why the change ?)
WolfOfSahara's avatar
I never knew this kind of stuff happens to other than insects and fungi, pretty cool! Well... not for the crabs!
The-Episiarch's avatar
It's actually really common in the natural world.

It pretty much happens to every living thing you can think of; parasitoid wasp hijacking caterpillars so that even after they are eaten alive from the inside they will act as body guard for the developing wasp cocoon, fungi that make plant grow "fake flowers" to help it spread its spores, fluke larvae that secrete chemicals into the brain of the fish they are infecting to make dance and attract the attention of fish-eating birds (the fluke's next host)...just to name a few - there are endless examples of parasite sequestering their host's behaviour (you can read about them at my blog here: [link]

It is so common that in fact, you can say it's pretty normal state of existence...right this moment you're probably surrounded by plants and animals that are actually under the spell of their parasites...
WolfOfSahara's avatar
Have you seen the new game Last of Us? It's based on this phenomenon!
The-Episiarch's avatar
From what I have seen of Last of Us, it seems to be based more closely on the cordyceps parasitic fungus which infect ants - I wrote about that fungus in a blog post here: [link]
Banvivirie's avatar
Neat!
Would you like maybe to submit this piece to our contest at BioIllustration? It fits one of the prompts and le deadline is upon us. :meow:
The-Episiarch's avatar
Thanks :)
Sure. I think I have to join the Bioillustration group to submit - so I've just done that. I'll submit it as soon as my request has been approved.
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