Kraienhemke, Lea (2015) Stable isotope fractionation rate in the carnivorous jellyfish Mnemiopsisleidyi (Agassiz, 1865). (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 26 pp.

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Abstract

Jellyfish have increased in abundance over the past decades in many marine food webs simultaneously with overfishing, and have also colonized new ecosystems as invasive species. The most prominent example may be provided by the comb jelly Mnemiopsis leidyi (Agassiz 1865), which is a highly successful invasive species in the Black, North and Baltic Sea. However, the feeding ecology of many species, including M. leidyi, is not well characterized, which makes it difficult to assess ecosystem impacts. Stable isotope (SI) analysis has emerged as an important method to gain insights into the trophic ecology of organisms. In this context, the fractionation rate, which is the difference in isotopic values between a consumer and its food sources, is an essential parameter to interpret SI values. My thesis had the objective to provide the first estimate of fractionation rates for carbon (C) and nitrogen (N) SI in M. leidyi, as foundation for much needed feeding ecology studies on this species. Specifically, a 2 week laboratory feeding experimentwith two experimental diets with controlled C and N isotopic valueswas done to determine the fractionation rate of M. leidyi. The experimental diets consisted of the brine shrimp Artemia salina (A. salina) and the copepod Acartia tonsa(A.tonsa)as its prey. A fractionation rate of M. leidyi fed on A. salina was calculated as 1.75‰ ± 0.75‰ for δ15N and 0.74‰ ± 0.95‰ for δ13C. The fractionation in δ15N is on the low end of values reported for other organisms, which often fall into the range of 3-4‰, demonstrating the importance of using species specific estimates. The δ13C estimate lies within the range of 0-2‰ often found in other organisms.The fractionation rate of M. leidyi fed on A.tonsa could not be calculated due to an early crash of the cultures. As lipid content of organisms can affect their δ13C values, different mathematical lipid correction methods were applied, and the corrected and uncorrected values were compared to gain insights into the magnitude and potential usefulness of the respective corrections were tested. In summary, in providing the first fractionation rate estimate for M. leidyi, this master thesis provides the basis for future studies on tropic ecology of M.leidyi using SI analysis.

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