Mechanics and energetics of medusan jet propulsion

A theoretical model is developed to describe the mechanics and energetics of medusae which swim by jet propulsion. The model leads to four generalizations: (i) The acceleration reaction, not drag, is the dominant fluid force experienced by swimming medusae. (ii) There exist behaviors such as contraction time and relative duration of the relaxation of contraction phases which maximize the velocity and efficiency and minimize the cost of locomotion during normal swimming. Also, escape performance is maximized with certain behaviors. Existing data for medusae show that many operate near these optima. (iii) The ratio of efficiency to cost of locomotion is a reasonable measure of swimming behavior and provides a means to study the relationships between swimming behavior and size, (iv) For medusae of a particular shape exhibiting a particular swimming behavior there exists a unique size which maximizes the ratio of efficiency to cost of locomotion.

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