Isotopic evidence for size-based dietary shifts in the jellyfish Cyanea nozakii in the northern East China Sea

Cyanea nozakii is a common bloom-forming Scyphomedusa in coastal waters of China. To understand the diet of this jellyfish and its trophic relationship with other zooplankton groups, stable isotope δ13C and δ15N values of C. nozakii in the bloom area within the northern East China Sea (ECS) in August 2017 were analyzed. We examined: first, the size-based variation in C. nozakii stable isotope values, second, the contributions of different food sources to the C. nozakii diet, and possible size-based dietary shifts in individual C. nozakii and, third, the contribution of gelatinous prey to the C. nozakii diet. Isotope values of differently sized C. nozakii ranged from −18.81 to −16.88‰ for δ13C and from 8.23 to 10.46‰ for δ15N. A significant change in δ15N values implies that the trophic position and the diet of C. nozakii changed as body size increased. The MixSIAR model indicated that C. nozakii underwent a dietary shift during growth, from a seston-dominant diet in smaller medusae to a zooplankton-dominant diet in larger medusae. Copepods were an important food source for C. nozakii, irrespective of size. The mean proportions of gelatinous zooplankton increased significantly in the C. nozakii diet with increasing C. nozakii diameter, suggesting that larger C. nozakii consumed more gelatinous prey. The diet of larger C. nozakii included 9.54% Nemopilema nomurai and 9.53% small medusae. The results suggest that intraguild predation among these jellyfish exist in the northern ECS.