Ontogenetic diet shift in the euphausiid Euphausia pacifica quantified using stable isotope analysis

Abstract

The euphausiid Euphausia pacifica is an omnivorous species of plankton that has been implicated in important food web processes such as the lengthening of food chains and the restructuring of community dynamics and energy transfer. We used carbon and nitrogen stable isotopes (delta C-13 and delta N-15) to quantify the ontogenetic diet shift in E. pacifica in the East Sea (Japan Sea or Sea of Japan). Analyses revealed that delta N-15 values of adults (4.17 +/- 0.10%) were significantly lower than those of juveniles (5.40 +/- 0.30%), reflecting a shift in the diet of E. pacifica adults towards copepod Neocalanus cristatus carcasses, which have been suggested to drift throughout the year in the epipelagic layer of the East Sea. This suggests a trend toward increasing detritivory with maturity, which is not consistent with the results of previous conventional approaches to diet analysis in E. pacifica. Our results should be useful to modelers concerned with quantifying the effects of E. pacifica on energy transfer in marine food chains.