Methods to collect, preserve, and prepare elasmobranch tissues for stable isotope analysis

Stable isotope analysis has the potential to expand our understanding of elasmobranch ecology. However, elasmobranchs share unique traits (i.e., retention of urea, lack of adipose tissue, cartilaginous skeletons) that require modified preparation techniques. Alternative tissue collection and preservation methods would allow sampling from ichthyology collections and at remote locations. We compared different collection, preservation, and preparation techniques to identify treatments that yielded robust isotopic data. Blood components collected in tubes coated with lithium heparin (an anti-coagulant) were not isotopically distinct from blood collected in no-additive tubes. Compared to frozen muscle, ethanol-treated muscle had altered delta C-13 values, but similar delta N-15 values. Finally, we removed lipids and urea with petroleum ether and deionized water, respectively. Although untreated and treated muscle had similar amino acid compositions, treated muscle preferentially lost N-14 and had greater C:N ratios. These results indicate that urea affects isotope ratios and that water treatment removes urea without altering muscle protein composition. Although not exhaustive, our study begins to address the need for elasmobranch-specific methods.