Differential parasite (Trematoda) encapsulation in Gammarus aequicauda (Amphipoda)

Because resistance to parasites usually has a cost for host species, it is theoretically expected that, in case of multi-infection, host immune respon ses should vary according to the levels of parasite pathogenicity. The crus tacean gammarid Gammarus aequicauda is the second intermediate host of 4 tr ematode species. Three of these parasites always encyst in the abdomen of g ammarids and have no particular effect on the host. However, 1 of these spe cies is sometimes able to encyst in the cerebroid ganglia of the gammarid a nd strongly alter its behavior in a way that increases its predation risk b y aquatic birds, the definitive hosts. In accordance with the hypothesis th at the level of parasite pathogenicity influences the likehood and the degr ee of host reaction. cases of melanization in our gammarid collection almos t exclusively concern the cerebral metacercariae. Our results also indicate that this melanization is able to cancel the behavioral alterations induce d by the parasite, suggesting that the cause of the manipulation is not the physical presence of metacercariae in the brain.