O’GRADY, S/P*; DEARING, M/D; University of Utah; University of Utah: Isotopic insight into host-endosymbiont relationships

Nitrogen isotopes have been widely used to investigate trophic pathways in ecological systems and research has shown an isotopic enrichment of 2-5� with trophic level increases. Few studies have used isotopic relationships to examine ecological systems within an animal. We examined the relationship between two species of gut nematodes, Physaloptera retusa and Parapharyngodon riojensis, and their lizard hosts. Physaloptera retusa is a known stomach parasite, whereas Pa. riojensis is free-living and may feed on intestinal contents, rather than host tissue. Based on the established trophic level enrichments, we predicted an enrichment of ~3� for parasitic Ph. retusa and an enrichment of ~6� for free living Pa. riojensis compared to host tissue. We measured ¹⁵N values of food, digesta, gut tissue, and nematodes of three lizard species: a wild insectivore (Liolaemus koslowskyi), a wild herbivore (L. buergeri), and a captive herbivore (Phymaturus antofagastensis) on a control diet. The average enrichment value for Pa. riojensis compared to gut tissue, 6.59�, supports the contention that this nematode is not parasitic. Food to tissue enrichment (&epsilon_food-tissue) for captive herbivores was 1.38�, &epsilon_{digesta-tissue} of wild herbivores was 1.71�. Small &epsilon_{digesta-tissue} values may be due to the inclusion of bacteria, cells, and food in digesta samples, but small &epsilon_food-tissue are unexplained. There was no significant difference in &epsilon_{tissue-nematode} for captive and wild herbivores, 6.45� and 6.73� respectively, and both values were significantly greater than &epsilon_{tissue-nematode} for wild insectivores: 1.25�. These results suggest that isotopic fractionation in a host-endosymbiont system is lower than that of traditional food webs. Additionally, the larger enrichment of Pa. riojensis relative to known parasites suggests they may not be parasitic.