A new study has shown that infection by parasites might be enhancing cannibalism in shrimps. The results suggest that parasitism may cause the host to increasingly take to cannibalistic behaviour.
Parasitism is described as the interaction between two organisms whereby the parasite inhabits the host’s body, feeding on it or deriving some other benefit such that the parasite gains something while the host loses something. In some extreme cases, the parasite affects the host’s behaviour so much that it is driven to gory death, or suicide, or even to increased cannibalism. A new study has focused on the latter, exploring the effect of parasitism on a species of shrimp pertaining to the rate at which the individuals affected feed on others of their kind.
Cannibalism is part of the reality of life on Earth – more than 3 000 species have been documented to indulge in the consumption of their own fellow members. This implies that feeding on conspecifics comes with a wide range of benefits. Intraspecific cannibalism boosts growth, promotes survival and, at other times, eliminates competitors. It is to be noted that none of this comes without a cost. Parasites are essentially harmful to hosts. Some of them can modify the rate of predation. Researchers of the new study meant to analyse the effects of parasitism on cannibalism: would it be similar to how predation is affected?
The research focused on a parasite known as Pleistophora mulleri specific to a freshwater shrimp called Gammarus duebeni celticus.
90 % of the shrimp are hosts to the parasite. What makes of this pair even more interesting is that the only way of transmission of the parasite is the consumption of an infected shrimp which can be either alive or dead.
The behaviour of infected and uninfected adult male and juvenile shrimps was analysed by the team. Their observations revealed that infected shrimps fed on the juveniles to a greater extent than the uninfected ones. In fact, the parasitised shrimps fed on twice as much of others of their species as the unparasitised ones.
The lead author, Mandy Bunke, interpreted the observations as follows:-
Parasitism might be increasing the demand for food as a result of the larger burden on the host that has to feed itself and unwillingly feed the parasites it is carrying. Therefore, this could be causing the shrimps to indulge more in cannibalism. Moreover, being infected by parasites greatly decreases one’s chances of survival, for instance, by compromising one’s ability to catch prey. In this case, consuming smaller conspecifics might be the easier way out.
Another finding entailed the choice of uninfected adults. The latter preferred consuming uninfected juveniles at large. However, infected shrimps did not discriminate between infected juveniles and uninfected ones. This suggests that the otherwise-healthy shrimps consume their ‘food’ and simultaneously avoid infection.