Nitrogen isotope fractionation and amino acid turnover rates in the Pacific white shrimp Litopenaeus vannamei

摘要

Fluctuations in an animal's trophic position (TP) can indicate changes within an ecosystem. Traditionally, bulk tissue or whole animal nitrogen (N) isotope analysis has been used to determine an animal's TP, but there are limitations to the application of this approach. Amino acid compound-specific isotope analysis (AA-CSIA) can be used to determine TP based on the difference between the δ~(15)N values of certain amino acids, commonly glutamic acid and phenylalanine. The rate at which ~(15)N is incorporated into amino acids is largely unknown, and this may affect the accuracy of relative TP estimates especially during periods of ecosystem change or ontogenetic changes in diet. To study amino acid turnover rates, the diet of the Pacific white shrimp Litopenaeus vannamei was changed in the laboratory from an unlabeled diet (7.2‰) to a ~(15)N-enriched diet (24‰), and animals were sampled over the course of 7 wk. AA-CSIA revealed that turnover times of amino acids vary considerably, with some amino acids having much more rapid turnover than others. The TP of L. vannamei (using δ~(15)N values of glutamic acid and phenylalanine) over the course of the experiment remained constant, and shrimp TP averaged 2.14 ± 0.15 (mean ± SD, n = 7) relative to its diet, which was as expected. Our results indicate that the δ~(15)N values of glutamic acid and phenylalanine can be used to accurately determine TP even while shrimp are undergoing a change to a diet with a different δ~(15)N value.