An experimental study on variations in stable carbon and nitrogen isotope fractionation during growth of Mysis mixta and Neomysis integer

The trophic isotope fractionation of mysids, in response to a change in the isotopic composition in their diet, was examined in the laboratory. Field-caught Mysis mixta and Neomysis integer had similar delta(13)C values, ranging between -23.1 and -21.5 parts per thousand. They also had similar delta(15)N values, and for both species, this value increased with body size. Experimental starvation did not change the isotopic composition. To test for metabolic fractionation of isotopes, the mysids were fed different diets, newly hatched Artemia and detrital Enteromorpha. The isotopic composition was monitored in abdominal muscle tissue, exoskeleton, and feces over 12 weeks. The delta(13)C composition in muscle tissue had not reached an isotopic equilibrium with the diet at the end of the experiment, while the exoskeleton came into equilibrium with the food in 2-3 weeks. Muscle was enriched in N-15 relatively to the food, +3.6 parts per thousand for Artemia and +2.7 parts per thousand for Enteromorpha. The isotopic composition in muscle, exuviae, and feces may form a basis for diet reconstruction of mysids. The feces delta(13)C and delta(15)N values mirror the diet over the last few hours, exuviae delta(13)C values represent nutrients metabolized 2-3 weeks ago, and muscle tissue integrates the isotopic signal over a relatively long period.