Compression of trophic discrimination in ¹⁵N/¹⁴N within amino acids for herbivorous gastropods

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Nitrogen isotope ratios of amino acids have been widely used to estimate the trophic position of organisms in ecological food webs. However, un-explainable compression/expansion of the trophic discrimination factor (TDF) has been reported in diverse consumer species. In particular, significant compression has been reported in herbivorous gastropods and bivalves, resulting in the estimated trophic position of them is lower than 2.0. In the present study, we report three diverse TDF for the gastropods Turbo sazae (7.5 ± 0.5‰), Haliotis discus (5.3 ± 1.0‰), and Lyncina vitellus (3.0 ± 0.7‰), as TDF found in H. discus and L. vitellus is considerably smaller than that generally found in many species (7.6 ± 1.3‰). The compression of TDF, however, well corresponds to the amount of proteinous mucus released from these species, implying that a part of the metabolic flux of amino acids is used for the production of proteinous mucus. Therefore, we suggest that understanding of TDF for consumer species with respect to ‘balance of metabolic flux’ between deamination of amino acids (i.e., energy production) and construction of own proteins including additional proteinous materials is required, to enhance accuracy in the application of nitrogen isotope ratios of amino acids to the food web studies.