The crab-castrating parasite that zombifies its prey

The crab-castrating parasite that zombifies its prey
Body-snatching crabs is not just for humans. Credit: Chesapeake Bay, CC BY-NC

Meet Sacculina carcini – a barnacle that makes a living as a real-life body-snatcher of crabs. Unlike most barnacles that are happy to simply stick themselves to a rock and filter food from the water, Sacculina and its kin have evolved to be parasitic, and they are horrifyingly good at it.

The microscopic larva of Sacculina seeks out an unsuspecting crab using specialised sensory organs. It then settles on a part of the crab where its armours is most vulnerable, usually on the membrane at the base of one of the crab's hair (called a setae).

The larvae then transforms itself into a kind of living hypodermic syringe (called a kentrogon). This syringe stabs the base of the crab's hair and injects the next stage of the parasite – a microscopic blob called the vermigon – into the crab's bloodstream. This blob will eventually grow into a parasite that takes over the crab's entire body.

The body of the fully mature Sacculina is unrecognisable as a barnacle (or any animal for that matter) – it consists of a part called the interna which looks more like the roots of a plant than any animal. Its tendrils spread throughout the crab's insides and the only part of the parasite which is visible on the outside is the externa – the female reproductive organ which protrudes from the crab's abdomen.

The crab-castrating parasite that zombifies its prey
The nightmare begins - how Sacculina larva infecting a crab. Credit: Tommy Leung

Sacculina takes over the host in both body and mind – it castrates the crab, then turns it into a doting babysitter that grooms and aerates the barnacle's brood, tending the next generation of baby-snatchers as if they were its own babies. Lest you think Sacculina is alone in its nightmarish ways, it is just one genus in an entire order of barnacles called Rhizocephala (the "root head").

No babies, no food

A recent study found the effects these parasites have on the host's behaviour also affect the rest of the ecosystem. On the coast of South Carolina lives the flatback mud crab (Eurypanopeus depressus), where it is infected with a species of rhizocephalan call Loxothylacus panopei. Usually, the mud crab has an omnivorous diet and sometimes feeds on , using their claws to pry open the shells. But that are infected with L. panopei lose their appetite for such fare.

When confronted with a pile of mussels, uninfected crabs treat it as an all-you-can-eat seafood buffet, and eat as much as they can without hesitation. The more mussels they are presented with, the more they eat. But no matter how many mussels you offered to crabs infected with L. panopei, they simply eat one and call it a day. The parasitised crabs also took longer to get their act together and this seems to be related to the size of the parasite – the larger the parasite has grown, the longer the crab takes to start digging into a mussel.

Based on a field survey of the estuary where the study took place, the researchers concluded that about a fifth of the crab at that location were infected with L. panopei. Given the effects that L. panopei has on a crab's appetite for shellfish, it seems that the mussels might have an unlikely ally in the form a parasitic barnacle. The finding of this study share some parallels with a species of muscle-wasting parasite that curbs the appetite of an otherwise ravenous freshwater shrimp which has become invasive in parts of Europe and the UK.

Thus Sacculina and its kinds are more than just body-snatchers – their impact extends beyond their hosts and affects the rest of the environment. Ecosystems are compose of complex networks of biological interactions, and mediating them are the which are often overlooked or ignored. But their effects on the ecosystem are certainly there – if you know what to look for.

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May 30, 2014
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May 30, 2014
Fascinating. Reminds me of the zombie ants and wasps that are controlled by a fungus.

No mention is made regarding whether this parasite makes the crab unsuitable for human consumption though. Also it says the parasite castrates the crab. Does this mean only males are affected? If not what happens to the females?

Parasitism truly is a wonder of evolution. Freaky!!

May 30, 2014
"The crab-castrating parasite that zombifies its prey"
Feminists crabs?

May 30, 2014
That is a very complex behaviour. What could have been the evolutionary pathway?
We could of cause be entirely mistaken about the arrow of Time and it is the Future that causes the present. We have cause and effect the wrong way round.
In which case the complex future animal guided the development of it's ancestors.

May 30, 2014
Having read this article, I feel like I've just watched another Alien sequel...

Life, vastly predating art imitating life...

May 31, 2014
Kind of like the ultimate dieting solution. (Tongue in cheek) for all who suffer from obesity, think there'd be much of a market for these if they could be tweaked to work in humans?

Thinking about dystopian futures, imagine if we could employ such parasites in humans for a wide range of behavior modification? Permanently eliminating criminal behavior would make an easy in for allowing such therapy. Down a steep slippery slope from that point.

May 31, 2014
Already been done. Fluoride in the water supplies, the wells, of the remote groups they wanted to influence. It took the will and intelligence out of them. A real world WWII program by the Nazi's. Fact. Interesting how it came to the west. Long story there, abuot projects and paperclips.

Same for saltpeter in the food of the incarcerated in many a western jail. Helps keep them under control.

Next, imagine an oversize pharmaceutical industry, running rampant....For the heck of it...Lets call them I.G. Farben, as a genesis point....

May 31, 2014
They left out the best part of the story.

The parasite has it's own hyper parasite, a highly modified copepod that looks like a white grape attached to the body of the sacculina.

It does the same thing to the sacculina that the sacculina does to the crab.

Jun 01, 2014
"he arrow of time is about the most obvious of phenomena - to the intelligent observant. One can lead a donkey to water but ya' can't make 'em think."

Perhaps a few books of Quantum phyics? But then Ya cant read nor write propper.

Jun 02, 2014
You see, this is why i don't eat sea food...

Jun 02, 2014
I'd think that since the parasite is also a crustacean, it's fleshy protuberance would be identical to crab meat when cooked.

In fact I bet it is, as there is probably no effort to NOT use parasitized crabs in the food industry when they process millions of crabs into the lump crab meat we can buy at supermarkets.

For that reason, considering how big these things can be in relation to the crab, an infected crab would have a much higher ratio of meat to shell, because the parasite is all soft tissue.

I'm surprised there isn't an effort to farm crabs and infect them to get the meatiest crabs possible LOL

Jun 02, 2014
The word that should have been used instead of castrate is....


Castrate is not a synonym for sterilize, which is actually what the parasite does.

Castrate makes it sound like it only affects male crabs leaving the female crabs able to reproduce which is not the case.

Both male and female crabs are effectively sterilized by the parasite which does this by radically altering the crab's hormonal balance.

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