Remarkable vertical shift in residence depth links pelagic larval and demersal adult jellynose fish

Deep-sea fish show diverse migratory behaviors across depths at different life stages. The historical residence depths of jellynose fish Ateleopus japonicus and Ijimaia dofleini (Ateleopodidae) were reconstructed from otolith microstructures and isotopic compositions. δ18O values in the otolith core areas ranged from −0.5 to −1.3‰ among individuals, suggesting that larval and post-larval stages lived in the mixed layer (50-200 m). Otolith growth increment widths surged for 10-30 rings around 300-600 µm from the core, indicating a fast-growth phase during the early post-larval stage. Fish then migrated downward to 350-800 m depth at about 2 months of age, possibly during the post-larval metamorphosis to the juvenile. Otolith growth increments became narrower and otolith δ13C values increased from −5 to −1‰, suggesting a lower growth and metabolic rate when the fish experienced colder water during the downward migration. After arrival at the deepest waters, the fish then migrated upward to the continental margin or upper slopes where the adults persistently resided. A translucent otolith zone was formed after the residence depth shift from the deepest waters to shallower depths, indicating a transition from pelagic to bathydemersal life on the continental shelf or break. The down-and-up shift in residence depth of jellynose fish represents an indirect settlement process to the adult residence depth, which might be associated with a unique post-larval stage moving offshore before the downward migration. The results filled the gap of vertical distributions of jellynose fish from pelagic larvae near the sea surface to the bathydemersal adult dwelling on the continental shelf break.