The unbearable lightness of shrimp

It is, we sometimes find; sitting here eating doughnuts and drinking sweet tea, easy to forget the innate cruelness of Nature. To us, the barbarism that 99.9% of species on this planet operate under seems a very distant truth. 

And it is easy to see why; life, for us at least, is rarely threatened (…despite the Editor’s frequent overtones to the contrary). The most distress we encounter ordinarily comes at the prospect of running out of jam-filled and having to fall back on ring.

When it comes to the tyrannical cruelty of nature, Darwin had it right (…obviously. It’s the big D we are talking about). The ‘fit’ – as in the most suited to their environment, not the ones that can do the most pull-ups – survive, the ‘weak’ die. And not quietly in their sleep with their family and friends gently weeping at their bed sides –they die a proper natural death. A teeth baring, claw slashing, disease wracked horror show of a death. A death that hammers home the fact that ‘natural causes’ means something decidedly less genteel to species other than Homo sapiens.

In a letter to his friend, the botanist and explorer Joseph Hooker, Darwin wrote: What a book a devil’s chaplain might write on the clumsy, wasteful, blundering, low, and horribly cruel work of nature!

Cruel indeed. Gruesome even. Beyond even the plot line of the most slash-heavy of slasher movies. Not convinced? Then let us introduce you to one of the most fantastical examples of Nature’s cruelty.

In an attempt to understand the delicate ecological balance between two competing species of freshwater shrimp in the apparent idyll of the Irish waterway system, a team from the University of Leeds uncovered something surprising. Writing in the journal Royal Society Open Science, they report that in the great battle for resources between the native Gammarus duebeni celticus and the invasive species Gammarus pulex, there is, in fact, a third player. A spore forming, unicellular organism known as Pleistophora mulleri.

So far so ordinary. But, it is how this miniature game changer influences the ecological balance that shows off its darker side. The team found that P. mulleri – no larger than a human red blood cell – is in fact a parasite, and a particularly dastardly one at that. Once it works its way into the musculature of the indigenous shrimp, it seems to be able to significantly increase cannibalism. Now, as we know Nature is harsh – and in line with that, consumption of juveniles by adults turns out to be a normal feature of the shrimp’s feeding patterns. But, shrimp infected with the parasite ate twice as many of their own kind as uninfected animals.

Not only that, infected shrimp became more voracious, taking much less time to consume their victims. Ravenous and cannibalistic – P.mulleri has made the erstwhile mild mannered freshwater shrimp (…ok, ok ‘mild mannered’ apart from the odd spot of ‘background’ cannibalism) the perfect host to pass on the parasite.

So, how exactly do they pull off this trick? Some clever biochemical interference of behavioural brain circuitry? That is unclear at this stage, but it looks as though it could be much simpler, it could all be down to pure hunger.

Mandy Bunke, a PhD student at the University of Leeds explains: “Although the parasite is tiny there are millions of them in the host muscle and they all rely on the host for food. This increased demand for food by the parasites may drive the host to be more cannibalistic.”

So, cannibalism AND parasitism. Here, evolution has offered up a twofer of truly horrific proportions. Darwin, we think, would approve…