Carbon translocation within land snails affects the carbon isotopic fractionation

Stable carbon isotope composition (δ¹³C) of land snail materials is widely used as a proxy for reconstructing past vegetation and environmental changes. Interpretation δ¹³C data is challenging due to the complexities of snail physiology. This study cultured Lissachatina fulica (Bowdich, 1822) snails to investigate δ¹³C of their soft tissues and excrement under various plant types and dietary carbonate content. Our results show that snails primarily assimilate organic matter (OM), with limited use of dietary carbonates. δ¹³C of organic matter in excrement correlates positively with that in the diet, but exhibits deviations due to variable carbon isotopic fractionation during assimilation. Tissue δ¹³C (δ¹³C_tissue) is positively related to the δ¹³C of OM (δ¹³C_OM) in the diet, varying by turnover rates, with faster turnover rate, such as the digestive gland, exhibiting more negative δ¹³C_tissue values. Moreover, carbon isotopic fractionation between tissues and diet (Δ¹³C_tissue-OM) differs based on plant type. Snails fed with C₃ plants show positive Δ¹³C_tissue-OM values, whereas those fed with C₄ plants exhibit negative values. These Δ¹³C_tissue-OM values are far from equilibrium (Δ¹³C_tissue-OM = 0) with conditions of rapid snail growth. Our findings indicate that the snails fed with C₃ and C₄ plants result in distinct carbon isotopic fractionation, leading to the deviation in δ¹³C between plants and land snail materials (e.g., excrement, tissues, shells and shell-bound organic matter). A comprehensive model of carbon metabolism and isotopic fractionation in snails is proposed including the assimilation process from diet to soft tissues and excretion. This study highlights the importance of physiological factors such as growth rates and turnover rates when interpreting δ¹³C of land snail materials for paleoenvironmental reconstructions.