In a new study published in the journal Royal Society Open Science, scientists from the University of Leeds, Queen’s University Belfast and Stellenbosch University in South Africa have investigated the effects of a tiny parasite on the cannibalism rate of its host, the freshwater shrimp Gammarus duebeni celticus. The results show that the parasite not only significantly increases cannibalism among infected shrimp but makes them more voracious.
Freshwater shrimp Gammarus duebeni celticus. Image credit: University of Leeds.
“Cannibalism is actually fairly common in nature. Our work is the first study to ask if cannibalism is affected by being parasitized,” said Dr Alison Dunn of the University of Leeds, UK, who is the senior author on the study.
Dr Dunn and co-authors said that although consumption of juveniles by adults is a normal feature of the shrimp Gammarus duebeni celticus’ feeding patterns, shrimp infected with Pleistophora mulleri ate twice as much of their own kind as uninfected animals.
They attacked juvenile shrimp more often and consumed them more quickly than did 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 first author Mandy Bunke, a PhD student at the University of Leeds.
“The parasite is quite debilitating,” Dr Dunn added. “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.”
The scientists also found that uninfected adult shrimp were less likely to cannibalize infected juvenile shrimp than uninfected juveniles.
“The parasite is passed to its new host either when it dies and is eaten by another shrimp, or when one shrimp cannibalizes another,” Dr Dunn explained.
“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. 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.”
Although there is evidence that parasites can affect human behavior, the current study does not suggest any link between parasites and human cannibalism. Recent studies have 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,” Dr Dunn said.
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Mandy Bunke et al. Eaten alive: cannibalism is enhanced by parasites. Royal Society Open Science, published online March 18, 2015; doi: 10.1098/rsos.140369
