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First published online May 21, 2007
Journal of Experimental Biology 210, 1868-1873 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.003772

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Morphological diversity of medusan lineages constrained by animal–fluid interactions

John O. Dabiri^1,*, Sean P. Colin² and John H. Costello³

¹ Graduate Aeronautical Laboratories and Bioengineering, California Institute of Technology, Pasadena, CA 91125, USA
² Environmental Sciences, Roger Williams University, Bristol, RI 02809, USA
³ Biology, Providence College, Providence, RI 02918, USA

^* Author for correspondence (e-mail: jodabiri{at}caltech.edu)

Accepted 11 March 2007

Cnidarian medusae, commonly known as jellyfish, represent the earliest known animal taxa to achieve locomotion using muscle power. Propulsion by medusae requires the force of bell contraction to generate forward thrust. However, thrust production is limited in medusae by the primitive structure of their epitheliomuscular cells. This paper demonstrates that constraints in available locomotor muscular force result in a trade-off between high-thrust swimming via jet propulsion and high-efficiency swimming via a combined jet-paddling propulsion. This trade-off is reflected in the morphological diversity of medusae, which exhibit a range of fineness ratios (i.e. the ratio between bell height and diameter) and small body size in the high-thrust regime, and low fineness ratios and large body size in the high-efficiency regime. A quantitative model of the animal–fluid interactions that dictate this trade-off is developed and validated by comparison with morphological data collected from 660 extant medusan species ranging in size from 300 µm to over 2 m. These results demonstrate a biomechanical basis linking fluid dynamics and the evolution of medusan bell morphology. We believe these to be the organising principles for muscle-driven motility in Cnidaria.

Key words: locomotion, biomechanics, fluid dynamics, medusae

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