Scientists are only beginning to understand the role of parasites in animal behavior, but now that they are looking, examples seem to be everywhere. An international team of researchers have found that a tiny parasite appears to play a significant role in cannibalism among freshwater shrimp. In addition, the shrimp made the animals much more voracious eaters.

Researchers from the University of Leeds, Queen’s University Belfast and South Africa’s Stellenbosch University specifically looked at the parasite Pleistophora mulleri and its impact on the freshwater shrimp Gammarus duebeni celticus.

“Cannibalism is actually fairly common in nature. Our work is the first study to ask if cannibalism is affected by being parasitised,” said Dr Alison Dunn in a statement.

Dunn is a Reader in Evolutionary Biology in the University of Leeds’ Faculty of Biological Sciences, and led the new study published in Royal Society Open Science.

The findings show that, although consumption of juveniles is a normal behavior for the shrimp, those affected by the parasite were much more enthusiastic about it. Infected shrimp ate juvenile shrimp twice as often and ate much more quickly than uninfected shrimp.

“Although the parasite is tiny–similar in size to a human red blood cell–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,” said Mandy Bunke, a PhD student at the University of Leeds and key researcher for the study.

“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. 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,” added Dr. Dunn.

The study also found that infected juvenile shrimp were less likely to be consumed by adults.

“The parasite is passed to its new host either when it dies and is eaten by another shrimp, or when one shrimp cannibalises another. But we observed that uninfected shrimp avoid parasitised food and that is good for the shrimp as it means that they can obtain food through cannibalism but still avoid parasitic infection. Infected shrimp don’t avoid infected juveniles. They consume infected and uninfected juveniles. This may be is because they are more hungry or because they are already infected so there is no incentive to avoid eating infected juveniles,” said Dr. Dunn.

Dunn added, however that the parasite does not necessarily have the same impact on humans and other animals that it does on the shrimp.

“Our research does not suggest any link between parasites and human cannibalism. There is evidence that parasites can affect human behaviour. A study led by Dr Glenn McConkey, also of the University of Leeds Faculty of Biological Sciences, has shown that the parasite Toxoplasma gondii directly affects the chemistry of the human brain. However, cannibalism for the shrimp, unlike in humans, is a significant source of food even in uninfected animals. It seems unlikely that a parasite would be under evolutionary pressure to influence cannibalism in humans,” said Dr. Dunn.

In addition to understanding feeding behavior and the role of the parasites, the research may also explain the decline of the shrimp population. Gammarus duebeni celticus is currently under threat in Irish waterways from Gammarus pulex, a different shrimp species from Great Britain. The researchers believe that the parasite may be partially responsible for weakening the native species.