Consistent isotopic differences between Schistocephalus spp. parasites and their stickleback hosts.

Parasite-host systems show markedly variable patterns in isotopic fractionation: parasites can be either depleted or enriched in ¹⁵N and ¹³C as compared to their hosts. However, it remains unknown whether isotopic fractionation patterns are similar in comparable parasite-host systems from markedly different ecosystems. Results of this study show that large-sized Schistocephalus spp. endoparasites are consistently depleted in ¹⁵N (by on average -2.13 to -2.20 ‰) as compared to their nine-spined stickleback Pungitius pungitius and three-spined stickleback Gasterosteus aculeatus hosts. The differences between parasites and host for both δ¹⁵N and δ¹³C were consistent in both study systems despite marked biogeographical differences between the study localities. Although the stable isotope values in general were strongly correlated between the hosts and their parasites, Schistocephalus specimens occupying the same nine-spined stickleback host showed sometimes substantial individual variation in δ¹³C. This might be due to selective use of different carbon sources, or different metabolic or feeding rates. Further studies on selective feeding, physiology and metabolism of parasites are needed to better understand the role of parasites in the structure and functioning of aquatic food webs.