Are lipids always depleted? Comparison of hydrogen, carbon, and nitrogen isotopic values in the muscle and lipid of larval lampreys

Stable isotope ratios in organisms can be used to estimate dietary source contributions, but lipids must first be accounted for to interpret values meaningfully. Lipids are depleted in heavy isotopes because during lipid synthesis light isotopes of carbon (C-12) and hydrogen (H-1) are preferentially incorporated. Prior work in larval lampreys has noted unusual lipid effects, which suggest lipids are enriched in the heavy isotope of carbon (C-13), but still depleted in the heavy isotope of hydrogen (deuterium; H-2); nitrogen, a relatively rare element in lipids, has not been identified as being as sensitive to lipid content. Our objective was to determine if stable isotope ratios of hydrogen, carbon, and nitrogen behaved as expected in larval lampreys, or if their lipids presented different isotopic behavior. The delta H-2, delta C-13, and delta N-15 were measured from the muscle of four lamprey species before and after lipid extraction. In addition, muscle of least brook lamprey (Lampetra aepyptera) was collected every three months for a year from two streams in Maryland. Isotopic ratios were measured in bulk and lipid-extracted muscles, as well as in extracted lipids. The difference between muscle samples before and after lipid extraction (Delta delta H-2, Delta delta C-13, Delta delta N-15) was positively related to lipid proxy (%H or C:N ratio) and were fit best by linear models for Delta delta H-2 and Delta delta N-15, and by a non-linear model for Delta delta C-13. The difference between lipid-extracted muscle and lipid delta C-13 (Delta(ML)delta C-13) was negative and varied between months (ANOVA, F-3,F-53 = 5.05, p < 0.005). Our work suggests that while lipids are often depleted in C-13, this is not a universal rule; however, the depletion of H-2 in lipid synthesis appears broadly true.