| Summary: | Scientific Committee on Antarctic Research Open Science Conference (SCAR 2020), 3-7 August 2020 Doris kerguelenensis is an Antarctic heterobranch mollusc feeding on several demosponges and hexactinellid sponges. To assess the contribution of several sponge species to its diet at Deception Island we used stable isotopes of C and N, as well as fatty acids. We examined the demosponges Axinella crinita, Dendrilla antarctica, Hemigellius pilosus, Kirkpatrickia variolosa, Mycale acerata, Sphaerotylus antarcticus, and Haliclona sp. We also analyzed the macroalgae Desmarestia anceps and Himantothallus grandifolius, and phytoplankton. D. anceps was the most 13Cdepleted one and phytoplankton the most enriched. The δ13C values of sponges and D. kerguelenensis laid in between. The δ15N values of A. crinita, D. antarctica, M. acerata and Haliclona sp. suggested a trophic position between 2-3, and for H. pilosus andS. antarcticus between 3-4. Kirkpatrickia variolosa was in between the two groups. The δ15N values of D. kerguelenensis revealed a trophic position from 3-4. All sponges and D. kerguelenensis had high levels of 15:0 and EPA and low levels of arachidonic acid, largely differing from macroalgae. Fatty acid profiles of A. crinita, D. antarctica, K. variolosa, M. acerata, S. antarcticus, and Haliclona sp. were similar to phytoplankton, but D. kerguelenensis and H. pilosus were enriched in 17:0, 20:1n9 and 20:2. D. kerguelenensis, A. crinita, D. antarctica, K. variolosa, Haliclona sp. and S. antarcticus shared a long chain fatty acid absent in phytoplankton and macroalgae. The overall evidence suggests that A. crinita, D. antarctica, K. variolosa and Haliclona sp. are the most likely prey of D. kerguelenensis at Deception Island Peer reviewed
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