Effects of preservation methodology on stable isotope compositions of sea stars

Rationale: Stable isotope analysis is used to investigate the trophic ecology of organisms and, in order to use samples from archived collections, it is important to know whether preservation methods alter the results. This study investigates the long-term effects of four preservation methods on isotopic compositions and isotopic niche parameters of sea stars.

Methods: We assessed the effects of preservation method (freezing, drying, formaldehyde, ethanol) and duration (0, 1, 3, 6, 9, 12, 24 months) on the stable isotope ratios of carbon, nitrogen and sulfur of sea star tissues. Isotopic ratios were measured using continuous-flow elemental analysis and isotope ratio mass spectrometry. We also monitored the evolution of commonly used ecological metrics (isotopic niche parameters) throughout the experiment.

Results: Clear changes in δ¹³ C values were observed for samples stored in formaldehyde and ethanol. None of the preservation methods had significant or consistent effects on δ¹⁵ N values. Formaldehyde preservation induced a decrease in δ³⁴ S values. All these changes could be mitigated using correction factors. Isotopic niche parameters slightly changed over time when computed with δ¹³ C and δ¹⁵ N values, but inconsistent variations occurred when computed with δ¹³ C and δ³⁴ S values.

Conclusions: Overall, these results show that preservation may affect the stable isotope ratios of sea stars. Correction factors can be used to mitigate the effects of the preservation method on stable isotope ratios. Isotopic niche parameters are overall unchanged. Consequently, in most cases, museum samples are suitable for calculation of isotopic niche parameters.