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The incorporation of δ13C and δ18O from respired CO2 to tests and whole calcified tissues (WCT) of the sea urchin Lytechinus variegatus was investigated in 120 individuals to elucidate the influence of diet type on the calcification process. Sea urchins were raised during 4 mo in controlled seawater tanks using 3 different diets (δ13C; δ18O) (means ± SE): seagrass (−10.2 ± 0.1‰; −15.2 ± 0.3‰), red macroalgae (−17.8 ± 1.4‰; −21.6 ± 0.8‰), and a formulated diet (−21.5 ± 0.04‰; −25 ± 0.4‰). Individuals fed the formulated and red macroalgae diets were depleted in both δ13C and δ18O compared to those fed seagrass. The isotope composition of skeletons mirrored that of organic carbon (δCo) and δ18O in diets, but contributions of dietary carbon were higher for urchins fed formulated (40.2 ± 1.2% in test and 34.8 ± 0.4% in WCT) or macroalgae diets (40.38 ± 1.1% in test and 32.1 ± 0.4% in WCT) than for those fed seagrass (29.1 ± 1% in test and 29.9 ± 0.7% in WCT), concurrently with greater growth rates and distinctive rates of fractionation. Differences between test and WCT contributions among diets could not be explained by patterns of biomass allocation across calcified structures (i.e. similar test to spines+lantern ratios for all diets). This suggests that individuals fed red macroalgae or formulated diets increased the amounts of dietary carbon going into their tests, whereas individuals fed seagrass did not. Overall, we identified diet type as a new factor in the process of carbonate deposition that could influence species’ responses to changes in ocean chemistry. We suggest that the reconstruction of sea urchin paleodiets might also be possible from fossil records. Test structures may be particularly useful for this purpose, because they accumulate high contributions of metabolic carbon that enhance the detection of differences in the isotopic signatures of diets.

δ13C and δ18O signatures from sea urchin skeleton: importance of diet type in metabolic contributions