Low lipid and urea effects and inter-tissue comparisons of stable isotope signatures in three nearshore elasmobranchs

Stable isotope analysis of carbon (δ¹³C) and nitrogen (δ¹⁵N) is a common tool used to examine aspects of elasmobranch biology and ecology; however, accurate ecological interpretation of stable isotope values requires knowledge of lipid and urea dynamics, and the variable turnover rates of different tissue types. Here we examined lipid and urea dynamics and inter-tissue comparisons of stable isotope values in 3 nearshore elasmobranch species, the nurse shark Ginglymostoma cirratum, southern stingray Hypanus americanus, and the Atlantic chupare stingray Styracura schmardae. Chemical extraction had no significant effect on δ¹³C and δ¹⁵N values of nurse shark muscle, and southern and chupare stingray fin, suggesting negligible lipid and urea components associated with these tissues. For nurse sharks, δ¹³C values were higher in muscle compared to dermis and δ¹⁵N was lower. The causes of this variability are underpinned by the metabolic variability between tissue types, the physiological function of which remains undetermined. Finally, we observed a significant relationship between muscle and dermis δ¹³C and δ¹⁵N, providing the first inter-tissue isotopic correction for nurse sharks. The results provide insight into lipid and urea dynamics, and aid sample preparation and ecological interpretation of stable isotope data in these taxa.