Jellyfish are known to achieve simple and efficient locomotion among other underwater species. Propulsion takes place by converting the muscular force used to contract the bell into forward thrust. The conversion ratio, as well as the propulsion performance, depend on a variety of factors: the bell geometry, the swimming kinematics, the stroke frequency, and the vortex dynamics. These give rise to two prevailing models of thrust generation, termed jet propulsion and rowing (or paddling) propulsion. Both are achieved by contraction of the muscle fibers lining the subumbrellar cavity. Jet propulsion is known to provide a high peak velocity, whereas rowing propulsion guarantees a low cost of transport. Although these propulsion mechanisms have been extensively studied, both numerically and experimentally, the correlation between locomotion performance, bell shape, and muscle activation pattern across the scales has not been fully understood. The present study exploits a fluid-structure interaction model to draw these correlations. A key novelty consists in the muscle activation model: the active strain approach allows to enforce a prescribed fiber shortening to replicate the subumbrellar muscle contraction [1]. All investigated shapes are endowed with the same space-time contraction pattern, such that a kinematically fear comparison is obtained. The resulting locomotion and vortex dynamics is modelled by means of a finite-differences/immersed boundary approach [2]. It is shown that the shift from jet propulsion to rowing propulsion can take place for different bell aspect ratios, depending on the Reynolds number. REFERENCES [1] Nitti, A., Torre, M., Reali, A., Kiendl, J., & de Tullio, M. D. (2023). A multiphysics model for fluid-structure-electrophysiology interaction in rowing propulsion. Applied Mathematical Modelling, 124, 414-444. [2] Nitti, A., Kiendl, J., Reali, A., & de Tullio, M. D. (2020). An immersed-boundary/isogeometric method for fluid–structure interaction involving thin shells. Computer Methods in Applied Mechanics and Engineering, 364, 112977.