EVALUATING THE PREVALENCE OF FUNGAL FARMING IN THE DIET OF THE MARSH PERIWINKLE LITTORARIA IRRORATA USING STABLE ISOTOPES

Salt marsh ecosystems are some of the most productive on Earth and are in decline worldwide. Grazing by the marsh periwinkle Littoraria irrorata plays an important role in limiting salt marsh productivity. The presence of L. irrorata shells in archaeological shell middens provides an opportunity to investigate historical changes in ecosystem-structuring trophic cascades relevant to salt marsh conservation efforts. To understand this trophic system better, we must resolve the prevalence of fungal farming versus marsh cordgrass grazing within the diet of L. irrorata. To this end, we performed a preliminary analysis of N and C stable isotope compositions using bulk soft tissues of L. irrorata collected from a Wilmington, NC salt marsh. We compared snail signatures to the published stable isotopic compositions of salt marsh vegetation, including the marsh cordgrass Spartina alterniflora (living, senescing, and dead), epiphytic microalgaes, benthic microalgaes, and macroalgaes. Published estimates of S. alterniflora’s isotopic composition show consistent values between years and geographically across the southeastern United States permitting the use of published values. Assuming L. irrorata has average C and N fractionation factors (0 to +1‰ and +3.4‰, respectively) the δ¹³C and δ¹⁵N signatures quantified herein are consistent with a diet of primarily dead and senescent S. alterniflora. While the lack of unique fungal isotopic signatures is a limitation of the current study, previous authors have attributed the depletion of δ¹⁵N in senescent S. spartina to the predominance of endophytic fungal nitrogen. Thus, the isotopic signatures of the L. irrorata herein are consistent with a diet consisting largely of fungus. Future studies will isolate the fungi present on the radulations of senescing S. alterniflora for isotopic or fatty acid analysis to further resolve the diet of L. irrorata to better inform historical studies of trophic dynamics in salt marsh systems.