Research data for: Halocline boundary layer restricts the vertical distribution of the box jellyfish Tripedalia cystophora Freiberg, Jan-Frederik Röhrdanz, Niels Kohlstedt, Hermann Bielecki, Jan research_data database https://doi.org/10.57892/100-359 https://opendata.uni-kiel.de/receive/fdr_mods_00000359 https://opendata.uni-kiel.de/receive/fdr_mods_00000359 doc-type:ResearchData ddc:500 Haloclines – sharp salinity gradients frequently formed after heavy rainfalls in coastal habitats – can act as barriers for weakly swimming plankton, but their biomechanical constraints on relatively adept swimmers, such as cubozoan jellyfish, remain unexplored. We examined the vertical distribution of Tripedalia cystophora before and after establishing an artificial halocline (35→22 PSU) in an experimental swimming arena. After halocline formation, animals repeatedly entered the gradient layer but did not ascend beyond its upper boundary, despite repeated upward trajectories towards the gradient layer, indicating no obvious avoidance response. A hydrodynamic model supported these observations, demonstrating that stratification drag markedly increases thrust dissipation. Thus, centimetre-scale haloclines impose physical constraints that prevent T. cystophora from accessing surface waters through reduced effective upward momentum, rather than through behavioural change. Because the underlying hydrodynamic principles are general, we propose a stratification hypothesis to explain how sharp density gradients shape the vertical distribution of some aquatic animals. 2026-05-05 eng https://creativecommons.org/licenses/by-sa/4.0/ Fulltext available at https://opendata.uni-kiel.de/receive/fdr_mods_00000359https://opendata.uni-kiel.de/servlets/MCRFileNodeServlet/fdr_derivate_00000352/Repository.7zhttps://opendata.uni-kiel.de/rsc/thumbnail/fdr_mods_00000359.png 2026-05-05 data set Research data for: Halocline boundary layer restricts the vertical distribution of the box jellyfish Tripedalia cystophora database Haloclines – sharp salinity gradients frequently formed after heavy rainfalls in coastal habitats – can act as barriers for weakly swimming plankton, but their biomechanical constraints on relatively adept swimmers, such as cubozoan jellyfish, remain unexplored. We examined the vertical distribution of Tripedalia cystophora before and after establishing an artificial halocline (35→22 PSU) in an experimental swimming arena. After halocline formation, animals repeatedly entered the gradient layer but did not ascend beyond its upper boundary, despite repeated upward trajectories towards the gradient layer, indicating no obvious avoidance response. A hydrodynamic model supported these observations, demonstrating that stratification drag markedly increases thrust dissipation. Thus, centimetre-scale haloclines impose physical constraints that prevent T. cystophora from accessing surface waters through reduced effective upward momentum, rather than through behavioural change. Because the underlying hydrodynamic principles are general, we propose a stratification hypothesis to explain how sharp density gradients shape the vertical distribution of some aquatic animals. en 500 10.57892/100-359 his Host institution aut Author Freiberg, Jan-Frederik 0000-0003-1572-0978 Freiberg Jan-Frederik Röhrdanz, Niels aut Author 0000-0001-6762-6962 Röhrdanz Niels Kohlstedt, Hermann aut Author 0000-0002-5181-8633 Kohlstedt Hermann Bielecki, Jan aut Author 0000-0001-5354-355X Bielecki Jan 2026-05-05 2026-05-05 2026-05-05 2026-05-15 434434223 fdr_mods_00000359https://opendata.uni-kiel.de/receive/fdr_mods_00000359 info:eu-repo/semantics/embargoedAccess