Effect of diet isotopic ratios on the δ13C and δ15N signatures of scallop-gut contents in a natural setting

The relationship between diet isotopic signatures and the extent of the tissue-diet isotope fractionation factors have been shown previously in controlled feeding studies, but this relationship remains poorly resolved for marine invertebrates in a natural setting. Using isotope analysis of gut contents, this study investigated the variability in tissue-diet isotopic fractionation factors (Delta delta C-13 and Delta delta N-15) between scallop tissues (i.e. muscle and digestive gland) and gut content in the Japanese scallop Mizuhopecten yessoensis (Jay, 1857) and assessed how the growth rate of juvenile scallops and gut content quality (as carbon to nitrogen (C/N) molar ratios) would affect the values of tissue-diet isotopic fractionation. Tissue-diet isotopic fractionation factors (Delta delta C-13 and triangle delta N-1) varied according to tissue, which averaged 1.7 parts per thousand and -0.1 parts per thousand for N in muscle and digestive gland, and 3.4 parts per thousand and 1.2 parts per thousand for C in muscle and digestive gland, respectively. Muscle-gut triangle delta N-1 isotopic factors negatively correlated with growth rate, suggesting that the juvenile delta N-15 signatures were strongly dependent on food quantity. The influence of diet quality on triangle delta N-1 was also evident, where triangle delta N-1 decreased as C/N molar ratios in gut contents increased. The wide difference in Delta delta C-13 and triangle delta N-15 from accepted standard values may be, in part, a consequence of using gut contents as a dietary proxy. Results of this study have implications for diet reconstruction studies of this scallop species reared under field conditions.