Tissue-specific Isotopic Incorporation Turnover Rates and Trophic Discrimination Factors in the Freshwater Shrimp Macrobrachium borellii (Crustacea: Decapoda: Palaemonidae)

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 (delta C-13 = 26.1 parts per thousand, delta N-15= 2.1 parts per thousand) for 45 days in attempt to standardize the shrimps' initial delta C-13 and delta N-15 values for subsequent experiments. Shrimps were then fed with another artificial diet (delta C-13 = 16.1 parts per thousand, delta N-15 = 15.8 parts per thousand) and the change in delta C-13 and delta N-15 was observed for a period of 97 days. The trophic discrimination factor (Delta) for delta C-13 was significantly higher in hepatopancreas (0.7 +/- 0.36 parts per thousand) than in muscle (-0.1 +/- 0.83 parts per thousand); however, the opposite was the case for delta N-15 (1.7 +/- 0.43 parts per thousand and 3.6 +/- 0.42 parts per thousand, respectively). In the hepatopancreas the mean residence time (tau) of C-13 was 26.3 +/- 4.3 days compared to a residence time of 16.6 +/- 5.51 days for delta N-15, whereas the r in muscle was 75.8 +/- 25 days for delta C-13 and 40 +/- 25 days for delta N-15. 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 (Delta N-15 around 3-3.5 parts per thousand and Delta C-13 around 0-1 parts per thousand), but our work provides evidence that these traditionally used values may vary in other tissues, as we found that in the hepatopancreas Delta N-15 is around 1.7 parts per thousand.

Automated summary

The study conducted a diet shift experiment on omnivorous shrimp Macrobrachium borellii, finding differences in carbon and nitrogen isotope discrimination factors in muscle and hepatopancreas tissues, as well as variations in average residence times of isotopes in these tissues.