An Assessment of Sample Preservation Methods for the Determination of Stable Carbon and Nitrogen Isotope Ratios in Mollusks

The aim of this study was to quantify the effects of preservation methods and preservation times on the stable carbon and nitrogen isotope values in the tissues of six mollusk species (Mytilus edulis, Crassostrea gigas, Ruditapes philippinarum, Acanthochiton seulschnochilon, Littorina brevicula, and Rapana venosa). To identify potential preservation effects on δ¹³C and δ¹⁵N values and to examine temporal changes in the effects of preservation, repeated analyses were carried out after 1 day, 5 days, 15 days, 1 month, 3 months, 6 months, and 12 months of preservation. The results showed that drying preservation was the most suitable method for preserving samples, while freezing and chemical preservation significantly affected the stable isotope values compared with those of the controls. The effects of preservatives on the tissues of different mollusk species were statistically significant for both δ¹³C and δ¹⁵N values. Shifts in the δ¹³C and δ¹⁵N values, due to freezing and chemical preservation, were higher in Acanthochiton seulschnochilon, Littorina brevicula, and Rapana venosa than in the other three species. The effects of preservatives on carbon isotope values were variable. In most cases, the shift of the δ¹³C values from the control samples were less than 1% for those of the treated samples. The δ¹³C and δ¹⁵N data from the preserved samples could thus be used in food web reconstruction studies. Further studies will be necessary, however, in order to elucidate the effects of preservation type and time on other species.