Fatty acids as the main energy source in polyps of the jellyfish (Cnidaria: Aurelia aurita) for survival strategy under overpopulation: insights from compound-specific isotope analysis

Abstract Jellyfish exhibit an extraordinary survival strategy in high-density populations that exceed the carrying capacity of ecosystems. It is particularly questionable how jellyfish can thrive in habitats where the concentration of toxic nitrogenous waste (i.e., NH 3 ) in the ambient water could be extremely high caused by high-density populations. However, physiological mechanisms of the survival strategy in jellyfish remain poorly understood, especially at the polyp stage, even though this stage is a key role in jellyfish population dynamics. In the present study, we investigated the trophic isotopic discrimination of amino acids and fatty acids in polyps of the jellyfish Aurelia aurita in controlled feeding experiments, to understand the metabolic flux of these organic substrates in the jellyfish physiology under high-density populations. Contrary to the general trophic isotopic discrimination observed in non-gelatinous organisms, the discrimination in the A. aurita polyps is close to zero for most amino acids (except for glycine and leucine), indicating a little activity of both biosynthesis and degradation of the amino acids. In contrast, the discrimination in the A. aurita polyps is considerably large for fatty acids (ranging from 18.4‰ (for C 18:0 ) to 50.5‰ (for C 18:2 , n -6), indicating a large activity of degradation of fats (and its components, fatty acids). Based on these results, we conclude that A. aurita polyps prioritize the use of fatty acids compared to that of amino acids to produce life energy, for minimizing the production of nitrogenous waste derived from the amino acid degradation (about 0.0 mg/L for NH 3 ) and maintaining the quality of ambient water, and therefore for allowing them to survive in high-density populations. Moreover, these findings suggest the presence of metabolic plasticity among marine organisms. This plasticity may reflect flexible survival strategies supported by different metabolic pathways. Therefore, integrating the trophic isotopic discrimination of both amino acids and fatty acids can be useful for understanding these physiological mechanisms, as well as for accurately illustrating energy transfer along food chains in ecosystems. Graphical Abstract