Abstract
Stable isotopes are important ecological tools, because the carbon and nitrogen isotopic composition of consumer tissue reflects the diet. Measurements of isotopes of individual amino acids can disentangle the effects of consumer physiology from spatiotemporal variation in dietary isotopic values. However, this approach requires knowledge of assimilation patterns of dietary amino acids. We reared leopard sharks (Triakis semifasciata) on diets of squid (Loligo opalescens; 1250 days; control sharks) or squid then tilapia (Oreochromis sp.; switched at 565 days; experimental sharks) to evaluate consumer-diet discrimination factors for amino acids in muscle tissue. We found that control sharks exhibited lower nitrogen isotope discrimination factors (∆15N) than most previous consumer studies, potentially because of urea recycling. Control sharks also had large carbon isotope discrimination factors (∆13C) for three essential amino acids, suggesting microbial contributions or fractionation upon assimilation. Compared to controls, experimental sharks exhibited higher ∆13C values for four amino acids and ∆15N values for seven amino acids, corresponding with differences between diets in δ13C and δ15N values. This suggests that not all amino acids in experimental sharks had reached steady state, contrary to the conclusion of a bulk isotope study of these sharks. Our results imply that (1) the magnitude of a shift in dietary δ13C and δ15N values temporarily influences the appearance of discrimination factors; (2) slow turnover of amino acid isotopes in elasmobranch muscle precludes inferences about seasonal dietary changes; (3) elasmobranch discrimination factors for amino acids may be affected by urea recycling and microbial contributions of amino acids.
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Acknowledgements
We thank the Institute of Marine Sciences and Long Marine Lab at UC Santa Cruz for help in acquiring and housing sharks; D. Casper for consultation and training throughout the experiment; volunteers (J. Adams, A. Bennett, M. Gorey, L. Krol, S. Perry, S. Rumbolt, A. Sjostrom, A. Thell, and C. Spencer) for the assistance with husbandry and sampling. Thank you to L. Germain, F. Batista, and M. McCarthy for preliminary CSIA results and encouragement to pursue this project. Funds for experimental infrastructure were from a National Science Foundation award to P. Koch (OCE 0345943) and from research funding provided to S. Kim (University of Kentucky). All applicable institutional and/or national guidelines for the care and use of animals were followed.
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SLK and PLK formulated the idea, SLK conducted the feeding experiment, all authors contributed to the development of the compound-specific stable isotope analyses, JPW, SLK, and SDN performed laboratory analyses, all authors interpreted the data, and JPW led the collaborative writing of the manuscript.
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Communicated by Donovan P German.
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Whiteman, J.P., Kim, S.L., McMahon, K.W. et al. Amino acid isotope discrimination factors for a carnivore: physiological insights from leopard sharks and their diet. Oecologia 188, 977–989 (2018). https://doi.org/10.1007/s00442-018-4276-2
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DOI: https://doi.org/10.1007/s00442-018-4276-2