Stable isotope and fatty acid tracers in energy and nutrient studies of jellyfish: a review

Studies of the trophic ecology of gelatinous zooplankton have predominantly employed gut content analyses and grazing experiments. These approaches record only what is consumed rather than what is assimilated by the jellyfish, only provide evidence of recent feeding, and unless digestion rates of different prey are known, may provide biased estimates of the relative importance of different prey to jellyfish diets. Biochemical tracers, such as stable isotopes and fatty acids, offer several advantages because they differentiate between what is assimilated and what is simply ingested, they provide an analysis of diet that is integrated over time, and may be useful for identifying contributions from sources (e.g., bacteria) that cannot be achieved using gut content approaches. Stable isotope analysis has become more rigorous through recent advances that provide: (1) signature determination of microscopic organisms such as microalgae, (2) analysis of dissolved organic carbon, and (3) improved quantification of relative source contributions. The limitation that natural tracer techniques require different dietary sources to have unique signatures can potentially be overcome using pulse-chase isotope enrichment experiments. Trophic studies of gelatinous zooplankton would benefit by integrating several approaches. For example, gut content analyses may be used to identify potential dietary sources. Stable isotopes could then be used to determine which sources are assimilated and modeling could be used to quantify the contribution of different sources to the diet. Analysis of fatty acid profiles could be used to identify contributions of bacterioplankton to the diet and, potentially, to provide an alternative means of identifying dietary sources in situations where the isotopic signatures of different potential dietary sources overlap. In this review, we outline the application, advantages, and limitations of gut content analyses and stable isotope and fatty acid tracer techniques and discuss the benefits of using an integrated approach toward studies of the trophic ecology of gelatinous zooplankton.