CANCELLED - Dilution effects under competition and intraguild predation in the snail-schistosome system

Background/Question/Methods

Schistosomes, the causative agent of human schistosomiasis, transmit between human and snail hosts via free-living life stages in freshwater. Biocontrol of Biomphalaria snails, the host genus for Schistosoma mansoni, has been attempted historically by introducing non-host snail species with the intention of outcompeting and extirpating Biomphalaria. However, competitor species now co-exist with Biomphalaria in many regions. Furthermore, the joint impact of competitors on the density and body-size structure of Biomphalaria populations remains unexplored. Body size is a key host trait for schistosome transmission because larger snails are more resistant to infection, but produce many more parasites if infected. In this study we first used a mesocosm experiment to characterize the impact of two competitor species, Melanoides tuberculata and Physa acuta, on Biomphalaria glabrata population size and body-size structure, egg laying, resource availability, and schistosome transmission potential. We used an additive design to generate communities with: (i) Biompahalaria only (ii) Biomphalaria and Melanoides (iii) Biomphalaria and Physa and (iv) Biomphalaria, Melanoides and Physa. All tanks were exposed to schistosomes. We collected population and individual-level data over 16 weeks. We then ran a laboratory experiment to test one potential ecological interaction between these three snail species: intraguild predation of Biomphalaria eggs.

Results/Conclusions

Tanks with only Biomphalaria snails showed a canonical pattern of an early increase in abundance followed by a crash in the population as resources became depleted. This pattern was muted in tanks with Melanoides, suggesting strong resource competition. Physa snails also reduced Biomphalaria abundance, but these communities also had the greatest resource availability, larger body sizes, and sustained egg laying over the course of the experiment. Combined, these findings suggested that Physa was predating Biomphalaria eggs. Subsequent laboratory experiments showed that Physa is a voracious predator of Biomphalaria eggs, while neither Melanoides nor Biomphalaria consumed significant numbers of Biomphalaria eggs. Collectively, these findings indicate that non-host snail species may influence Biomphalaria population dynamics via divergent mechanisms, with differing potential consequences for schistosome transmission.