Tissue-specific Isotopic Incorporation Turnover Rates and Trophic Discrimination Factors in the Freshwater Shrimp...

The interpretation of isotopic data in ecology requires knowledge about two factors: turnover rate and the trophic discrimination factor, which have not been well described in freshwater shrimps. We performed a 142-day diet shift experiment on 174 individuals of the omnivorous shrimp Macrobrachium borellii, measured their growth, and temporally serially sampled muscle and hepatopancreas tissue to quantify carbon and nitrogen incorporation rates and isotope discrimination factors. Shrimps were fed with artificial diets (δ¹³C = -26.1‰, δ¹⁵N= 2.1‰) for 45 days in attempt to standardize the shrimps' initial δ¹³C and δ¹⁵N values for subsequent experiments. Shrimps were then fed with another artificial diet (δ¹³C = -16.1‰, δ¹⁵N = 15.8‰) and the change in δ¹³C and δ¹⁵N was observed for a period of 97 days. The trophic discrimination factor (∆) for δ¹³C was significantly higher in hepatopancreas (0.7 ± 0.36‰) than in muscle (-0.1 ± 0.83‰); however, the opposite was the case for δ¹⁵N (1.7 ± 0.43‰ and 3.6 ± 0.42‰, respectively). In the hepatopancreas the mean residence time (τ) of ¹³C was 26.3 ± 4.3 days compared to a residence time of 16.6 ± 5.51 days for δ¹⁵N, whereas the τ in muscle was 75.8 ± 25 days for δ¹³C and 40 ± 25 days for δ¹⁵N. The rate of incorporation of carbon into muscle was higher than that predicted by allometric equations relating isotopic incorporation rate to body mass that was developed previously for invertebrates. Our results support ranges of traditional trophic discrimination factor values observed in muscles samples of different taxa (∆¹⁵N around 3‒3.5‰ and ∆¹³C around 0‒1‰), but our work provides evidence that these traditionally used values may vary in other tissues, as we found that in the hepatopancreas ∆¹⁵N is around 1.7‰.