Size-based changes in trophic ecology and nutritional quality of moon jellyfish (Aurelia labiata)

Despite their seemingly watery constitution, jellyfish are eaten by a diverse range of predators. However, while the role of jellyfish in marine food webs is gaining attention, the nutritional value of these gelatinous organisms and how it varies remain poorly understood. In this study, we aimed to connect jellyfish food web biology and nutrition by determining the relationship between jellyfish size, diet, and nutritional quality. We measured stable isotope (SI) and fatty acid (FA) profiles of particulate organic matter (POM), plankton size classes, and gelatinous zooplankton collected from a coastal site in British Columbia, Canada, in July and September 2019. Gelatinous zooplankton comprised 152 Aurelia labiata collected across both months (bell diameters = 19-225 mm), and 2 Aequorea victoria and approximately 30 Pleurobrachia bachei collected in July for comparison. According to FA trophic markers and chlorophyll a concentrations, the POM was a mix of phytoplankton, microzooplankton, bacteria, and detritus. According to delta N-15, POM was similar to 1 trophic level lower than plankton. All gelatinous zooplankton had low FA content (5-6 mu g/mg dry mass) compared with zooplankton (43-65 mu g/mg dry mass). A Bayesian mixing model (MixSIAR) was used to assess the contributions of POM and plankton to A. labiata diet, which demonstrated a size-based shift from 40% plankton at 19 mm to 70%-75% at 225 mm. Similarly, both delta C-13 and delta N-15 increased with size and A. labiata of 19 mm were similar to 1 trophic level lower than A. labiata of 225 mm. We also documented size-based changes in nutritional quality of A. labiata, where C:N decreased with A. labiata size, and essential FAs arachidonic acid (ARA) and docosahexaenoic acid (DHA) increased with size while eicosapentaenoic acid (EPA) did not change. A. labiata were elevated in ARA compared with other zooplankton, ranging from 1% to 9%. The changes in C:N and DHA of A. labiata with size mirrored the changes in their diet, but changes in ARA and EPA did not. Overall, these findings emphasize the importance of considering jellyfish size and taxonomy when evaluating nutritional pathways through jellyfish in marine food webs, and support that diet is one important driver of jellyfish biochemical composition.