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

Abstract

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.