Trophic discrimination factor of nitrogen isotopes within amino acids in the dobsonfly (Megaloptera: Corydalidae) larvae in a controlled feeding experiment.

The trophic discrimination factor (TDF) of nitrogen isotopes (N/N) within amino acids, between a stream-dwelling dobsonfly larva (: Megaloptera; Corydalidae) and its diet (chironomid larvae), was determined in controlled feeding experiments. Last-instar larvae of were collected from the Yozawa-gawa River, central Japan, and reared in the laboratory. After fed to satiation for 1 month, one group of larvae was each fed one living chironomid larva per day for 4 weeks, while a second group was starved for 8 weeks. The larvae were harvested at intervals and the nitrogen isotopic composition of glutamic acid (N) and phenylalanine (N) were determined to calculate TDF. The mean TDF of satiated and starved larvae were 7.1‰ ± 0.5‰ (= 3) and 7.3‰ ± 0.5‰ (=5), respectively. Thus, the TDF for larvae in this study was similar to that reported for other arthropods (approximately 7‰) and was independent of satiation or starvation. A previous study of wild larvae, based on the N and N values, estimated its trophic position (TP) as approximately 2.0 ± 0.1 (=5), a low value close to that of algivores, although they are generally characterized as carnivores (usually accepted as TP ≥ 3). The TDF for larvae suggests that their low TPs in nature were caused by incorporation of vascular plant-derived amino acids (with a different N profile from that of algae) and not by an unusually low TDF or by the effects of the satiation/starvation on amino acid metabolism.