The life-cycle of trematodes typically involves asexual reproduction in molluscs and the use of a secondary intermediate host for transmission (fish, amphibians...) to the final host where sexual reproduction occurs (birds). The production of cercariae larvae in the mollusc is considered critical for these parasites. Indeed, this production of thousands of clones determines an important part of the parasite's success to complete its life-cycle.
The trematode Microphallus sp. is widespread in New Zealand (although undescribed). It reproduces in the gonads of the aquatic snail Potamopyrgus antipodarum (see picture). Kopp and Jokela (2007, Oikos) showed that the presence of the great pond snail Lymnea stagnalis, following its introduction at the end of the 19th century, alters the transmission of Microphallus sp.: the presence of the introduced species reduces the ability of the parasite to enter in contact with its native host.
Indeed, the parasite has evolved to infect the native snail but the introduced species is not compatible with the parasite (i.e. Microphallus sp. can't reproduce efficiently in the introduced snail). Therefore, these introduced hosts constitute decoys for the parasite. This in turn decreases the probability of infecting the native P. antipodarum, a phenomenon called dilution effect. Overall, the study by Kopp and Jokela highlights the need to consider infection rates in a more realistic context, taking in account other species in the community as well as their interactions.
source for picture: http://www.indiana.edu
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