Comparison of ¹³C and ¹⁵N discrimination factors and turnover rates between congeneric crayfish Orconectes rusticus and O. virilis (Decapoda, Cambaridae)

Ecological applications of stable isotope analysis are dependent on knowing consumer-diet discrimination factors (Δ) and consumer metabolic turnover rates (m). We used an 80-day laboratory experiment to test for differences in the δ¹³C, δ¹⁵N and m of two species of crayfish (Orconectes rusticus and O. virilis) fed one of two diets (algae wafers and bloodworms). Over the course of the experiment, the δ¹³C and δ¹⁵N signatures of the crayfish approached equilibrium with those of their diets. We fit our data to a growth-based model and found δ¹³C, δ¹⁵N, and m to be largely indistinguishable between species, except in the case of δ¹⁵N and m of crayfish on the algae diet. We thus pooled parameters to calculate Δ¹³C (algae diet: 1.57‰ [95% confidence interval: 0.86–2.35]; bloodworm diet: 0.8‰ [0.14–1.55]) and Δ¹⁵N (bloodworm diet: 1.2‰ [0.32–2.11]), and used species-specific data to calculate Δ¹⁵N for the algae diet (O. rusticus: 2.54‰ [2.06–3.08]; O. virilis: 3.35‰ [2.53–4.51]). Our results provide values of stable isotope Δ and m for applications to crayfish, and offer a rare comparison of these values between two closely related species and to commonly used literature values.