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

Naoto F Ishikawa,Yoko Sasaki,Naohiko Ohkouchi,Yoshito Chikaraishi,Fumio Hayashi

doi:10.1002/ece3.2728

Abstract

The trophic discrimination factor (TDF) of nitrogen isotopes (15N/14N) within amino acids, between a stream‐dwelling dobsonfly larva (Protohermes grandis: Megaloptera; Corydalidae) and its diet (chironomid larvae), was determined in controlled feeding experiments. Last‐instar larvae of P. grandis 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 (δ 15NGlu) and phenylalanine (δ 15NPhe) were determined to calculate TDF. The mean TDF of satiated and starved larvae were 7.1‰ ± 0.5‰ (n = 3) and 7.3‰ ± 0.5‰ (n = 5), respectively. Thus, the TDF for P. grandis 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 P. grandis larvae, based on the δ 15NGlu and δ 15NPhe values, estimated its trophic position (TP) as approximately 2.0 ± 0.1 (n = 5), a low value close to that of algivores, although they are generally characterized as carnivores (usually accepted as TP ≥ 3). The TDF for P. grandis larvae suggests that their low TPs in nature were caused by incorporation of vascular plant‐derived amino acids (with a different δ 15N profile from that of algae) and not by an unusually low TDF or by the effects of the satiation/starvation on amino acid metabolism.

Highlights

The trophic position (TP) of an animal is an important dimension of its ecological niche in a given food web (Chase & Leibold, 2003)
The trophic discrimination factor (TDF) for P. grandis larvae suggests that their low TPs in nature were caused by incorporation of vascular plant-derived amino acids and not by an unusually low TDF or by the effects of the satiation/starvation on amino acid metabolism
This could be explained by (1) species-specific unusually low TDF, (2) reduction in the TDF associated with prolonged starvation inherent in their lifestyle, or (3) increased β value caused by incorporation of vascular plant-derived amino acids into the basal resources of the stream food web

Summary

| INTRODUCTION

The trophic position (TP) of an animal is an important dimension of its ecological niche in a given food web (Chase & Leibold, 2003). CSIA-AA estimated the TP of P. grandis larvae as 2.0 ± 0.1 (n = 5, based on β = −3.4‰), considerably lower than the value of TP ≥ 3 expected from its biological and functional position as a carnivorous animal (Ishikawa et al, 2014) This could be explained by (1) species-specific unusually low TDF, (2) reduction in the TDF associated with prolonged starvation inherent in their lifestyle, or (3) increased β value caused by incorporation of vascular plant-derived amino acids into the basal resources of the stream food web. To examine the above hypotheses (1) and (2), in this study we reared wild-caught P. grandis larvae in controlled feeding experiments in the laboratory, under satiated and starved conditions, and determined the δ15NGlu and δ15NPhe values to calculate TDF

| MATERIALS AND METHODS

| DISCUSSION