Effects of fluid preservation on sea star stable isotope compositions: How useful can museum collections be for trophic ecology studies?

Stable isotope analyses (SIA) of carbon (δ13C) and nitrogen (δ15N) are a common tool to investigate the trophic ecology of animals. After field sampling, animal tissues are usually preserved dried or frozen before subsequent analysis. However, museum samples, or samples not initially collected for SIA are often fixed in preservative solutions. These preservation methods may alter stable isotope ratios in animal tissues, and those effects should be quantified. Here, we investigated long-term effects (one year) of four preservation methods (freezing, alcohol, formaldehyde, drying) on δ13C and δ15N values of sea stars. We show that only formaldehyde preservation significantly affects δ13C values. However, after being altered, the mean δ13C value remained stable over time, suggesting that a correction factor may be used regardless of preservation time. δ15N values change across time for all methods except freezing but this may be the result of a higher intra-individual variability of δ15N values. The influence of preservation methods on estimation of isotopic niches size (parameterised as standard ellipse area) was also investigated. Bayesian estimations of standard ellipse areas (SEAB) of preserved samples were not different from those of fresh samples except for the samples preserved 6 months in alcohol. These preliminary results suggest that samples stored during one year in preservative fluids may be used for SIA in order to investigate the trophic ecology of sea stars. Future results will also help to determine whether the findings can be generalised to sea stars preserved for a longer time, such as those in museum collections.