Shrimp can become cannibals when they are infected by a certain breed of parasite, according to a new finding. Researchers at the University of Leeds, Stellenbosch University, and Queen's University Belfast made the announcement.

Pleistophora mulleri, a type of parasite that can infect freshwater shrimp from northern Ireland, drove cannibalism in the tiny crustaceans. These infections also caused these attacks to happen much more quickly than normal, researchers concluded.

"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," Mandy Bunke of the University of Leeds said.

Cannibalism is not uncommon in nature, and the researchers wanted to study how parasites affect the behavior. These shrimp, Gammarus duebeni celticus, frequently eat their own young,  but the study found that infection by Pleistophora doubled the attacks on juveniles. Attacks by healthy shrimp targeted healthy young more often than those who were infected by the microorganism. Adult crustaceans cannibalized infected and healthy juveniles at nearly equal rates, the study found.

"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... Perhaps cannibalism of smaller shrimp is the only way these sick animals can survive," Alison Dunn of the University of Leeds said.

Cannibalism provides an important source of nutrition for the shrimp, even in individuals who have not been infected by the parasites. Researchers theorize that the parasite may be driving cannibalism in the shrimp by causing them to be hungrier than normal. Adults who have been infected by the parasites may eat sick juveniles because there is no reason to avoid the microorganisms.

Gammarus duebeni celticus is being pushed out of its native waters by the introduction of an invasive species from Great Britain, Gammarus pulex. It is possible that the parasite and resulting cannibalism could be driving shrimp populations down, increasing the effect of the competing species.

This study does not provide any information about cannibalism among humans, and the effect of parasites on that practice. However, it is known that certain parasitic microorganisms can affect behaviors of human beings.

Cannibalism derives its name from the Spanish word Caníbales, the name given to a tribe in the West Indies that practiced the act. Today, the practice is rare, except in dire emergencies.

Analysis of the role played by Pleistophora in cannibalism among shrimp was profiled in the journal Royal Society Open Science.