Experimentally derived estimates of turnover and modification for stable isotopes and fatty acids in scyphozoan jellyfish

Abstract

Stable isotopes (SIs) and fatty acids (FAs) are biomarkers that are commonly used in ecology to investigate trophic relationships, energy pathways over time, and food quality. To use these trophic markers to model complex in situ data, accurate parameters should ideally be estimated from feeding studies performed in controlled laboratory settings. Two key parameters are turnover times and modification between trophic levels. However, food web studies for jellyfish that employ biomarkers typically apply generic parameter values because jellyfish-specific estimates are scarce. To address this, we implemented a controlled laboratory feeding study to measure turnover time and modification of FAs and SIs of carbon (δ13C) and nitrogen (δ15N) in Aurelia aurita and Chrysaora pacifica. Two diet switch experiments were conducted and resulted in three feeding scenarios: (i) Aurelia aurita and Chrysaora pacifica feeding on the same crustacean prey, Artemia, (ii) Aurelia aurita feeding on two different crustacean prey treatments, Artemia and Euphausia superba, and (iii) Chrysaora pacifica feeding on Aurelia aurita. Turnover time was variable among markers, ranging from 17 to 34 days in Aurelia aurita for FAs. We were unable to estimate SI turnover time for Aurelia aurita. In Chrysaora pacifica, half-life for δ13C (15 days) was similar to some FAs (8 to 15 days) but was half as long as δ15N (36 days). We estimated trophic enrichment factors for specific predator and prey combinations, including jellyfish feeding on crustacean zooplankton (Δδ13C = 1.19 ± 0.37‰ and Δδ15N = 2.09 ± 0.52‰) and jellyfish feeding on interspecific jellyfish (Δδ13C = 1.59 ± 0.69‰ and Δδ15N = 1.35 ± 1.70‰). Modification of FAs was widespread and should be considered when applying FAs to investigate jellyfish ecology. A pattern emerged in predators compared to their prey, where proportions of 18-carbon FAs decreased and proportions of 20-carbon FAs almost always increased. This suggests the possibility of a 2-carbon elongation pathway in jellyfish, similar to what has been found in other marine zooplankton. Total μg of FA per mg of sample dry weight in predators decreased over time when switched to prey with lower total FA than the initial prey. The parameter estimates provided here expand current applications of SIs and FAs for jellyfish trophic ecology, improve parameterization of jellyfish food web models, and ultimately expand our understanding of the complex role that jellyfish have in food webs.