LAUSR

Abstract Fatty acids (FAs) are vital biomolecules crucial for determining food quality for higher trophic levels. To investigate FA transfer and turnover time in predators, we conducted a diet switch experiment using jellyfish polyps. These polyps were fed food sources including Artemia sinica nauplii and FA‐manipulated copepod Pseudodiaptomus annandalei, maintained on distinct algal diets with varied FA compositions. Our findings reveal that copepods may have a strong potential to synthesize long‐chain polyunsaturated FA to maintain biochemical homeostasis when consuming low‐quality food. Consequently, the species‐specific fatty acid composition within plankton, combined with effects of seasonal environmental fluctuations and climate change, leads to changes in the FA composition of foundational food web components. These alterations create a complex “nutrient black box” effect as they propagate up trophic levels. Our study shows that jellyfish polyps fail to accumulate EPA and DHA but display high levels of ARA compared to their zooplankton and phytoplankton food sources, suggesting a potential association with dietary EPA and DHA through an unidentified pathway. Certain FA components indicate variations in the turnover time when polyps undergo a dietary shift. Understanding the trajectory of FA metabolism across the “phytoplankton–zooplankton” interface, along with its turnover time, provides crucial insights for modeling diet estimation of components within food webs.