Determination of δ13C and δ15N and trophic fractionation in jellyfish: implications for food web ecology

Application of stable isotope analysis (SIA) in jellyfish allows definition of trophic patterns not detectable using gut content analysis alone, but analytical protocols require standardization to avoid bias in interpreting isotopic data. We determined delta C-13 and delta N-15 in Aurelia sp. from the northern Gulf of Mexico (30A degrees 00'N, 89A degrees 00'W-30A degrees 24'N, 88A degrees 00'W) to define differences in stable isotope composition between body parts and whole body, the effect of lipid extraction on delta C-13 in tissues, and fractionation values from medusa to prey. The isotopic composition of bell and whole Aurelia sp. was not different. The increase in delta C-13 values after lipid removal suggested a correction is needed. To aid future analyses, we derived a correction equation from empirical data for jellyfish samples. Laboratory feeding experiments indicated medusae increased +4 aEuro degrees in delta C-13 and +0.1 aEuro degrees in delta N-15 compared to their diet. These results suggest protocols commonly applied for other species may be inaccurate to define Aurelia sp. trophic ecology. Because Aurelia spp. are commonly found in marine ecosystems, accurately defining their trophic role by use of SIA has implications for understanding marine food webs worldwide.