The preparation of jellyfish for stable isotope analysis

Gelatinous zooplankton are important predators, prey, and nutrient conduits within marine ecosystems. Information obtained from jellyfish stable isotope compositions can be invaluable to biological and environmental research and management. Protocols for best practice in preparing jellyfish for stable isotope analysis, however, require standardisation to provide consistently comparable data, with ecologically significant changes in values due to freezing reported in the literature. Jellyfish are easily sampled during standard marine fieldwork, and usually frozen before analysis. Here, mesoglea from freshly caught moon jelly, Aurelia aurita, were treated by thorough washing and/or freezing, and compared with untreated sections from the same individuals. Jellyfish were captured in July 2013 from Buckler’s Hard Marina in southern UK (latitude: 50.801, longitude: − 1.423). Isotope and element ratio changes of carbon and nitrogen composition due to the treatment of the mesoglea were quantified. Both washing and freezing elevated δ15N values, with washing also decreasing the variance observed in these values. Untreated mesoglea showed the lowest δ13C values. Carbon-to-nitrogen elemental ratio increased with both washing and freezing. These results imply the presence of a water-soluble, isotopically depleted nitrogenous component in fresh jellyfish mesoglea. The concentration of this component varies among individuals, and thorough washing or freezing are recommended to ensure consistent stable isotope analyses of jellyfish mesoglea. This study describes a methodology aimed at improving the consistency and repeatability of stable isotope analyses of jellyfish.