A non-destructive test was developed to measure the static mechanical properties of the locomotor structure (bell) in the hydromedusan jellyfish, Polyorchis penicillatus (Eschscholtz, 1829). A nonlinear stress-strain relationship was found, and the mean static structural stiffness of the bell was 150 N m−2. Visualization procedures that showed the natural changes in the geometry of the deformation of the bell were used to calculate the static modulus of elasticity of the mesoglea, and gave a modulus of 400 N m−2. Dynamic measurements on isolated samples of mesoglea gave a mean storage modulus of 1000 N m−2. The resilience of the material was about 58%. These data were integrated to imply that the dynamic structural stiffness of the bell is at least 400 N m−2. Attempts to measure the dynamic structural stiffness directly indicate that the dynamic stiffness of the intact bell lies between 400 and 1000 N m−2. All, or most, of the potential energy stored in the mesoglea during contractions of the bell is stored as strain energy in the radial mesogleal fibres.

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