Multiple isotope analyses of the pike tapeworm Triaenophorus nodulosus reveal peculiarities in consumer-diet discrimination patterns

Previous studies of dietary isotope discrimination have led to the general expectation that a consumer will exhibit enriched stable isotope levels relative to its diet. Parasite-host systems are specific consumer-diet pairs in which the consumer (parasite) feeds exclusively on one dietary source: host tissue. However, the small numbers of studies previously carried out on isotopic discrimination in parasite-host (Delta XP-HT) systems have yielded controversial results, showing some parasites to be isotopically depleted relative to their food source, while others are enriched or in equilibrium with their hosts. Although the mechanismfor these deviations from expectations remains to be understood, possible influences of specific feeding niche or selection for only a few nutritional components by the parasite are discussed. Delta XP-HT for multiple isotopes (delta C-13, delta N-15, delta S-34) were measured in the pike tapeworm Triaenophorus nodulosus and two of its life-cycle fish hosts, perch Perca fluviatilis and pike Esox lucius, within which T. nodulosus occupies different feeding locations. Variability in the value of Delta XP-HT calculated for the parasite and its different hosts indicates an influence of feeding location on isotopic discrimination. In perch liver Delta XP-HT was relatively more negative for all three stable isotopes. In pike gut Delta XP-HT was more positive for delta C-13, as expected in conventional consumer-diet systems. For parasites feeding on pike gut, however, the delta N-15 and delta S-34 isotope values were comparable with those of the host. We discuss potential causes of these deviations from expectations, including the effect of specific parasite feeding niches, and conclude that Delta XP-HT should be critically evaluated for trophic interactions between parasite and host before general patterns are assumed.