Research data for: 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.