Effects of high hydrostatic pressure on the mechanical performance and behavior of shallow-water jellyfish ( <i>Aurelia aurita</i> )

Hydrostatic pressure in the marine environment increases linearly with depth, and organisms at 1000 m experience pressures 100 times greater than those at sea surface level. Previous work has examined the effects of pressure on neuron and nervous system activity in some organisms, as well as the various biochemical adaptations of deep-water species. However, the effects of pressure on other biological tissues are not well understood. In this study, we took the shallow-water jellyfish Aurelia aurita and exposed it to pressures of up to 30 MPa (equivalent to 3000 m depth). We observed behavioral and kinematic changes that are likely due to mechanical effects of hydrostatic pressure on the swimming muscles and bell mesoglea. The pulsation rate of the bell was found to correlate with hydrostatic pressure, although the effect was small relative to the variability between individuals. Both the maximum contraction and relaxation rates of the bell were found to be significantly reduced at high pressure (30 MPa) relative to near-surface pressure (<1 MPa). The changes in pulse frequency and relaxation rate were both fully and immediately reversed upon release of pressure, but the change to contraction rate was not. Because bell contraction is controlled by muscle fibers and relaxation is controlled by elastic fibers in the mesoglea, the differential effects on contraction versus relaxation suggest that different tissues are affected differently by pressure. This opens the way for future work on how individual organisms can adapt to different environments.