RESUMO
Stable isotope ratios of carbon (13C/12C, δ13C) and nitrogen (15N/14N, δ15N) in snail-host tissue (the foot and hepatopancreas) and trematode parasites on two stages of their life cycle were analyzed. Trophic structure in co-occurring trematode larvae was examined in the following species: five species of cercariae (Echinoparyphium recurvatum, Hypodereum conoideum, Plagiorchis mutationis, Diplostomum chromatophorum and D. volvens) and two species of metacercariae (Cotylurus cornutus and Echinoparyphium recurvatum) within two closely related snail hosts Lymnaea stagnalis and L. tumida using stable isotope analyses. Snail and parasite sampling was conducted in a riverine portion of the Kargat River of the Lake Chany basin, in the south of Western Siberia (54 degrees 37'76"N, 78 degrees 13'07" E), in August 2009. Four out of five studied cercariae species were depleted in 15N as well as in 13C relatively to snail hosts tissues (foot and hepatopancreas), supporting our previously published data for Plagiorchis mutationis and Echinoparyphium recurvatum cercariae. Such fractionation of cercariae is untypical of the commonly observed relationship between consumers and their food. D. chromatophorum cercariae had demonstrated an insufficient enrichment in δ15N volume in relation to the hepatopancreas. Both two species of metacercariae (C. cornutus and E. recurvatum) showed a significant enrichment in δ15N volume relatively to the host tissue consumed (fractionation values ranging from 1.5 to 4 per thousand depending on the species). The differences in δ15N and δ13C volume between cercariae and metacercariae observed in this study illustrate the complexity of the host--parasite trophic relationships. Such isotopic differences between cercariae and metacercariae can probably be explained by selective consumption of specific amino acids or lipids or by changes in metabolism associated with the life cycle of the parasite. The present study represents the first comparative analysis of trophic relationships between the host and the endoparasite at different life cycle stages. It demonstrates the potency of the stable isotope analyses for understanding trophic relationships in multispecies parasite communities.
