Stable isotope discrimination factors and between‐tissue isotope comparisons for bone and skin from captive and wild green sea turtles (Chelonia mydas)

Abstract

RATIONALE The ecological application of stable isotope analysis (SIA) relies on taxa-and tissue-specific stable carbon (Δ13C) and nitrogen (Δ15N) isotope discrimination factors, determined with captive animals reared on known diets for sufficient time to reflect dietary isotope ratios. However, captive studies often prohibit lethal sampling, are difficult with endangered species, and reflect conditions not experienced in the wild. METHODS We overcame these constraints and determined the Δ13C and Δ15N values for skin and cortical bone from green sea turtles (Chelonia mydas) that died in captivity and evaluated the utility of a mathematical approach to predict discrimination factors. Using stable carbon (δ13C values) and nitrogen (δ15N values) isotope ratios from captive and wild turtles, we established relationships between bone stable isotope (SI) ratios and those from skin, a non-lethally sampled tissue, to facilitate comparisons of SI among studies using multiple tissues. RESULTS The mean (±SD) Δ13C and Δ15N values (‰) between skin and bone from captive turtles and their diet (non-lipid extracted) were 2.3±0.3 and 4.1±0.4 and 2.1±0.6 and 5.1±1.1, respectively. The mathematically predicted Δ13C and Δ15N values were similar (within 1‰) to the experimentally derived values. The mean δ15N values from bone were higher than those from skin for captive (+1.0±0.9‰) and wild (+0.8±1.0‰) turtles; the mean δ13C values from bone were lower than those from skin for wild turtles (–0.6±0.9‰), but the same as for captive turtles. We used linear regression equations to describe bone vs skin relationships and create bone-to-skin isotope conversion equations. CONCLUSIONS For sea turtles, we provide the first a) bone-diet SI discrimination factors, b) comparison of SI ratios from individual-specific bone and skin, and c) evaluation of the application of a mathematical approach to predict stable isotope discrimination factors. Our approach opens the door for future studies comparing different tissues, and relating SI ratios of captive to wild animals.

Regional Variation in Kemp’s Ridley Sea Turtle Diet Composition and Its Potential Relationship With Somatic Growth

“…Stable isotope mixing models require estimates of diet-tissue trophic discrimination factors (TDFs; )-the difference in isotopic ratios between consumers and their diet-to estimate the proportional contribution of different prey groups to consumer diets. As diet-bone TDFs have not been quantified for Kemp's ridleys or other primarily carnivorous sea turtles, we used dietbone TDFs estimated from dead, captive, juvenile green sea turtles (Chelonia mydas) ( 13 C = 2.1 ± 0.6, 15 N = 5.1 ± 1.1) (Turner Tomaszewicz et al, 2017b). Although these turtles were maintained on omnivorous diets composed of ∼56% animal matter (squid, shrimp, and fish) and ∼43% plant matter (lettuce) by weight, percent digestible N and C from animal protein was estimated to be 96.8 and 81.9%, respectively.…”

“…For our study, a growth-induced decline in 15 N values may have caused us to underestimate the proportional contribution of fish to turtle diets for faster growing individuals. In contrast, animals that consume large amounts of animal-derived proteins typically have higher 15 N values (Vander Zanden et al, 2012;Kurle et al, 2014;Turner Tomaszewicz et al, 2017b). A diet-induced increase in 15 N would therefore potentially have the opposite effect as growth on TDFs, causing an overestimation of the proportional contribution of fish to turtle diets for individuals that forage higher in the food web.…”

Quality control for modern bone collagen stable carbon and nitrogen isotope measurements

“…Although collagen has been analysed for decades in archaeology, collagen from a wider range of species and tissues has been increasingly studied in contemporary contexts (e.g. Dixon, Dempson, & Power, 2015; Guiry, Royle, et al, 2020; Skovrind et al., 2019; Turner Tomaszewicz, Seminoff, Price, & Kurle, 2017; Vokhshoori et al., 2019). In part, this is because collagen (Table 1) is the most abundant protein in many tissues that are routinely archived in biological and conservation programmes (e.g.…”

Beyond trophic morphology: stable isotopes reveal ubiquitous versatility in marine turtle trophic ecology

“…Further, we lack a comprehensive framework to compare stable isotope values across all combinations of sampled tissues and across all species. Although a few studies have proposed conversion factors for some pairs of tissues, they were typically within a single species and life stage (Ceriani et al, ; Kaufman et al, ; Tomaszewicz et al, ). Hence, there is no common currency that would permit standardised comparison across all tissues.…”