Indirect effects of herbicide on trematode proliferation in the freshwater snail host Potamopyrgus antipodarum

Freshwater ecosystems are often exposed to intense agricultural pollution, which can impact species interactions such as those between parasites and their hosts. I studied the effect of glyphosate (the active ingredient of a widely-used agricultural herbicide, Roundup®) on the proliferation and transmission of trematode parasites in the New Zealand mud snail, Potamopyrgus antipodarum. This ubiquitous and highly abundant snail serves as the first intermediate host to a wide diversity of trematodes. Trematode larval stages multiply within the snail to form free-living infective stages known as cercariae which then go on to infect native invertebrates, fish and birds. Earlier evidence suggested that herbicides from agricultural run-off might weaken the immune system of the snail and promote the within-snail multiplication of the trematode Telogaster opisthorchis. I tested the effect of long-term exposure to different levels of glyphosate on snail behaviour, cercarial production by three trematode species parasitic in P. antipodarum, and cercariae survival. Glyphosate had an effect on snail behaviour, however infection by an undescribed renicolid trematode did not. Snails exposed to the pollutant were hidden more than their conspecifics in the control treatment. Exposure of snails to glyphosate doubled, and in some cases tripled, cercarial output in three trematode species, i.e. the previously-mentioned renicolid, Coitocaecum parvum, and Apatemon sp.. In addition, survival time of renicolid cercariae was 1.57% greater when glyphosate was present at moderate concentrations. The more a parasite’s quality and quantity increase, the more likely we will see cascading effects on other hosts (fish, amphibians and molluscs). My results provided evidence that there are indirect effects from agricultural run-off on freshwater systems, and add weight to the pressure on the agricultural sector to limit the large-scale use of herbicide