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Mechanics of jet propulsion in a hydromedusean jellyfish DeMont, Malcolm Edwin


A non-destructive test was developed to measure the static mechanical properties of the locomotor structure (bell) in the hydromedusean jellyfish, Polyorchis penicillatus. A nonlinear stress-strain relationship was found, and the mean static structural stiffness of the bell was 150 N m⁻² . Observations that showed the natural changes in the geometry of the bell during deformation were used to estimate the static modulus of elasticity of the mesogleal material, 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 resilence of the material was about 58%. These data were integrated to infer that the dynamic structural stiffness of the bell is at least 400 N m⁻². Attempts to quantatitively measure the dvnamic structural stiffness imply that the dynamic structural stiffness must lie between 400 N m⁻² and 1000 N m⁻² All, or most, of the potential energy stored in the mesoglea during contractions of the bell is apparently stored as strain energy in the radial mesogleal fibers. The mechanical energy generated by the contraction of the subumbrellar swimming muscles to power the jet cycle was measured. This energy was experimentally partitioned into three components during the contraction. The algebraic sum of these components was taken to be the mechanical energy crenerated by the muscles during the iet cycle, and was between -5 -4 8.9 x 10⁻⁵ and 1.4 x 10⁻⁴. Energy from one of these components is stored as strain energy in the mesoglea and powers the refilling phase. The mesoglea can clearly act as an effective elastic structure to completely antagonize the contraction of the swimming muscles, and may be designed to function at some optimum. The mechanical significance of elastic energy storage systems in jet-propelled animals is discussed, and this significance is clearly displayed in Polvorchis. The unusual long duration action potential of the swimming muscles may have some mechanical significance with regard to the elastic energy storage system. It is suggested that the action potential of vertebrate cardiac muscle may have the same mechanical function. The locomotor bell of the hydromedusean jellyfish was modelled as a harmonically forced damped oscillator. The robustness of the model was tested and verified by comparing estimates of the work done during the contraction phase predicted by the model to analogous values measured in completely independent experiments. Data suggest that the animals swim at a frequency that is at or near the resonant frequency of the locomotor apparatus. The implications of this phenomenon to the mechanics and physiology of the system are discussed. If the swimming muscles force the bell at its resonant frequency, as opposed to a single contraction at the same rate of deformation, the amplitude of the oscillation will be increased by about 40%, and the energetic requirement for the cycle will be reduced by about 24% to 37% of the total cost of the cycle. The advantages of forcing the structure at its resonant frequency seem quite remarkable. Two aspects of the physiology of the swimming muscles are probably functionally related to this phenomenon and are discussed.

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