Diet-dependent δ13C and δ15N fractionation among sea urchin Lytechinus variegatus tissues: implications for food web models

Consumer-diet discrimination factors (Δ15N and Δ13C) are often applied without corroboration from laboratory experiments. Deviations in Δ15N and Δ13C that may occur from different diet types were quantified by raising 120 sea urchins Lytechinus variegatus in laboratory tanks on 3 different diets: seagrass Thalassia testudinum, red foliose macroalgae (Grauteloupia sp. and Palmaria palmata) and a mixed diet specifically formulated for L. variegatus. Patterns of δ13C and δ15N and resulting fractionation factors were then determined in muscle, gonad, gut, test organic matrix and whole individuals. Tissue δ13C values showed a strong positive association with δ13C of diets and estimates of absorption efficiency (R2 = 0.81). The seagrass diet consistently resulted in negative Δ13C values in all tissues (from −0.86‰ in muscle to −1.63‰ in gonad) and whole individuals (−1.19‰), whereas macroalgal and formulated diets showed positive values (0.11 and 0.19‰, respectively). Only individuals on the formulated diet clearly reached isotopic equilibrium for δ13C, suggesting that other lower quality diets may have resulted in more continuous reallocation of internal resources. δ15N values increased as the nitrogen content of these diets decreased (3.18, 1.21 and 0.82‰ for seagrass, macroalgae and formulated diets, respectively). Overall differences in the biochemical composition of diets and a robust relationship between δ13C and δ15N signatures suggest that protein quantity and quality could be central in driving isotope fractionation. The influence of macrophyte material type in the diet can be stronger than that of trophic level for both δ13C and δ15N, so further compound-specific isotope analyses are necessary to determine reliable values of Δ13C and Δ15N for ecological applications.