Accounting for parasites in aquatic communities: Community consequences of trematode-snail elemental mismatches

Background/Question/Methods

Parasites are ubiquitous members of ecological communities, but have only recently been recognized as key players in broad ecological interactions and ecosystem dynamics.  Trematodes are flatworm parasites that infect gastropods as intermediate hosts, often castrating and occupying significant amounts of within-shell gastropod biomass.  I explored the effects of trematodes on gastropod body elemental content, excretion N:P ratios, and egestion N:P ratios using field collections, experimental infections, and laboratory experiments.  The consequences of altered snail elemental ratios on snail-resource interactions were then investigated using a field survey of 50 sites in east-central Indiana and in an outdoor mesocosm containing Physa acuta and periphyton.

Results/Conclusions Infection rates of trematodes on field-collected freshwater snails ranged from 2 to 36%.  Infected snails had significantly higher body N:P than uninfected snails.  Trematode rediae and cercariae N:P ratios were lower than snail N:P ratios but did not differ among trematode taxa. Additionally, periphyton N:P ratios were positively related to snail infection rates.  In outdoor mesocosms, experimentally-infected Physa excreted higher N:P ratios than uninfected snails, resulting in significantly higher periphtyon N:P ratios in mesocosms with infected snails than in mesocosms with uninfected snails.  Thus, trematodes indirectly affected periphyton N:P by altering host snail excretion rates and content.  The indirect effects were stronger in nutrient-limited mesocosms, illustrating the importance of ecological context on parasite-community interactions.  Overall, these results indicate that trematodes modify snail-periphtyon interactions through a nutrient pathway and suggest that accounting for parasites may provide insight into important benthic processes.