This Parasite Turns Shrimp Into Cannibals!

Hypothetical Hank was having a good day, until he became infected. Now all he wants to do is eat his neighbor. Thankfully for us, Hank isn't the man to start our zombie apocalypse - he's just a shrimp. However, Hank is a strong example of how a tiny parasite can turn some species into cannibals.

A study recently published in the journal Royal Society Open Science details how the parasite Pleistophora mulleri may actually be one of the major factors that drive cannibalism in nature, as it was found to heavily increase instances of cannibalism among indigenous shrimp Gammarus duebeni celticus, and made infected shrimp much more voracious.

Alison Dunn, an evolutionary biologist at the University of Leeds who led the study, is quick to point out that aside from many species of mammals, "cannibalism is actually fairly common in nature."

Dunn and her colleagues wanted to find out why exactly that is, and they quickly determined that insatiable hunger caused by parasitism could be to blame.

But, how do they know? Many adult shrimp species are known to eat juveniles of their kind, either simply by accident, or perhaps because they just really wanted to know what junior tasted like.

In a close examination of feeding patterns of G. duebeni celticus, the research team found that while plenty of uninfected shrimp will gobble up the occasional wayward youngster, shrimp heavily infected with P. mulleri were twice as likely to commit cannibalism. Those infected were also found to far more frequently and voraciously attack easy-to-eat juveniles. Even infected juveniles reportedly attacked their peers when they grew more hungry. (Scroll to read on...)

"The parasite is quite debilitating. It takes over huge areas of the muscle, so instead of a nice transparent shrimp you get quite a chalky appearance because of muscles packed with the parasite," Dunn explained in a statement. "Interestingly, our group has also found previously that infected shrimp may be able to catch and eat less prey of other animal species. Perhaps cannibalism of smaller shrimp is the only way these sick animals can survive."

Still, it's not like fellow shrimp don't know to avoid a neighbor clouded with infection, even if they start looking like a tasty snack. Instead, they can choose to eat the healthy young - a way this parasitism can encourage cannibalism even among the uninfected.

"The parasite is passed to its new host either when it dies and is eaten by another shrimp, or when one shrimp cannibalizes another," Dunn said. "But we observed that uninfected shrimp avoid parasitized food and that is good for the shrimp as it means that they can obtain food through cannibalism but still avoid parasitic infection."

"Our research does not suggest any link between parasites and human cannibalism," she added reassuringly.

According to Dunn's team, even in the case of parasites can affect the human state of mind, there needs to be an evolutionary predisposition towards cannibalism in the first place for the behavior to be encouraged. Shrimp naturally commit cannibalism as part of a complex system of resource and population control. Humanity, on the other hand, has no such need for that drive.

However, this work does reveal the extent of parasites' influence on biological systems - helping experts understand how parasite-carrying invasive species can influence ecosystems in new and unexpected ways around the world.

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