Insights from stable isotope and fatty acid analyses on the diet of an Antarctic nudibranch

Reconstructing predator-prey interactions in a biological community provides a better understanding of species’ ecological roles and helps predict how changes in one component will affect the others. Doris kerguelenensis is an Antarctic heterobranch mollusc that feeds on sponges, according to previous gut content analyses. Here, stable isotopes of carbon and nitrogen (δ 13 C and δ 15 N) and fatty acids (FAs) were used to assess the relative contribution of several species of demosponges ( Axinella crinita , Dendrilla antarctica , Hemigellius pilosus , Kirkpatrickia variolosa , Mycale acerata , Sphaerotylus antarcticus , and Haliclona sp.) to D. kerguelenensis diet. Particulate organic matter and 2 macroalgae species were also analyzed to better characterize the diversity of sources fueling the food web. The δ 15 N values of the sponges suggest a trophic position of 2-3 for A. crinita , D. antarctica , M. acerata , and Haliclona sp. and of 3-4 for H. pilosus and S. antarcticus . Mixing models failed to resolve the nudibranch diet, either because the sponges were too enriched in 13 C or because the diet-to-tissue discrimination factor values available in the literature for gastropods are unsuitable for this nudibranch species. The high abundance of bacterial FA markers in H. pilosus , S. antarcticus , and K. variolosa , and their high δ 15 N values, rule them out as major prey for D. kerguelenensis . The overall evidence suggests that A. crinita , D. antarctica , M. acerata , and Haliclona sp. are consumed regularly by D. kerguelenensis but do not represent its only dietary sources. Hence, the analysis of biomarkers refutes the previously established role of D. kerguelenensis as a specialized spongivore.