Halocline boundary layer restricts the vertical distribution of the box jellyfish Tripedalia cystophora.

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.