Ontogenetic diet change of hadal amphipods in the New Britain Trench revealed by fatty acid biomarker and stable isotope ratio

The hadal zone (from 6 to 11 km depth) is one of the least explored habitats on Earth, and the knowledge about trophic ecology of hadal organisms is limited. Here, we analyzed fatty acid biomarker and stable isotope ratios of the amphipod species (Alicella gigantean) collected in the New Britain Trench (8225 m depth) in the southwestern Pacific Ocean. A total of 50 samples were divided into five developmental stages according to body length (1–129 cm): Stages I (juvenile), II (sub-adult) and III/IV/V (adult). Stages III, IV and V were further divided according to sex. Mean total fatty acid content for stage I (237.7 ± 105.6 mg g-1) was significantly lower than that for stages II to V (476.4 ± 49.0 mg g-1). The δ15N (14.7–18.4‰) positively correlated with the body length, whereas the EPA/DHA (C20:5ω3/C22:6ω3) ratio (0.09–0.51) negatively correlated with the body length. These results suggested an increase of trophic level with growth. Principal component analysis (PCA) separated major fatty acids into three clusters: (1) C18:1ω9, C16:1ω7, C14:1ω5 (indicative of carrion-based diet); (2) C14:0, C15:0, isoC15:0, C16:0, C17:0, isoC17:0, anteisoC17:0, C18:1ω7, C18:0, C20:1ω9, and C22:1ω9 (bacteria/zooplankton-based diet); and (3) C20:4ω6, C20:5ω3, C22:6ω3 and C22:5ω6 (benthic fauna/algae-based diet). The PCA result revealed that stage I-individuals consumed more bacteria/zooplankton debris, stages II to IV-individuals primarily preyed on carrion, while stage V-individuals had more diverse diet (benthic fauna/algae and carrion). Female amphipods had higher proportions of monounsaturated fatty acids, lower proportions of polyunsaturated fatty acids, and higher δ15N compared to male amphipods, suggesting that female individuals rely more on carrion from higher trophic organisms. This sex-related difference, however, diminished from stage III to V. Overall, our fatty acid and stable isotope data suggest an ontogenetic diet shift for hadal amphipods that may contribute to their success in the deepest ocean.