Stable carbon and nitrogen isotope discrimination in soft tissues of the leatherback turtle (Dermochelys coriacea): Insights for trophic studies of marine turtles

The trophic ecology of marine vertebrates has been increasingly studied via stable isotope analysis of body tissues. However, the theoretical basis for using stable isotopes to elucidate consumer-prey relationships remains poorly validated for most taxa despite numerous studies using this technique in natural systems. In this study, we measured stable carbon and stable nitrogen diet-tissue discrimination (Δdt) in whole blood, red blood cells, blood plasma solutes, and skin of leatherback sea turtles (Dermochelys coriacea; N = 7) maintained in captivity for up to 424 days and fed an isotopically consistent control diet with a mean C:N ratio of 2.94:1.00 and an energetic content of 20.16 ± 0.39 kJ g− 1 Dry Mass. We used a random-effect repeated measure model to evaluate isotopic consistency among tissue samples collected on days 276, 348, and 424. Both δ13C and δ15N remained consistent among sampling events in all tissues (all 95% posterior intervals for the slopes of a linear model included zero), indicating that all tissues had fully integrated diet-derived stable isotope compositions. Mean tissue-specific δ13C ranged from − 18.30 ± 0.16‰ (plasma solutes) to − 15.54 ± 0.14‰ (skin), whereas mean δ15N was from 10.06 ± 0.22‰ (whole blood) to 11.46 ± 0.10‰ (plasma solutes). The computed Δdt factors for carbon ranged from − 0.58‰ (plasma solutes) to + 2.25‰ (skin), whereas Δdt for nitrogen was from + 1.49 (red blood cells) to + 2.85 (plasma solutes). As the only discrimination factors available for leatherback turtles, our data will be useful for future interpretations of field-derived stable isotope data for this species. The inherent variability in Δdt values among individuals was low, which supports the value of these data for dietary reconstructions. However, it is important to note that tissue-specific discrimination factors for leatherbacks contrast with the widely accepted values for endothermic species (0-1‰ for C, 3-5‰ for N), and are also different from values established for hard-shelled turtles. This underscores the need for species- and tissue-specific discrimination factors before interpreting trophic studies of wild animals, including marine turtles.