PTV visualization of currents generated by upside-down jellyfish

\textit{Cassiopea} medusae, commonly known as upside-down jellyfish, are found in shallow, protected marine environments. These animals have a mostly sessile lifestyle, resting their bell on the substrate and pointing their oral arms upward facing the sunlight. \textit{Cassiopea} medusae periodically pulsate their bell to generate currents for suspension feeding, exchange and excretion processes. 2D PIV studies have shown the formation of a starting vortex during bell contraction that is pushed upward through the oral arms during bell relaxation. A continuous upward jet is generated above the oral arms during both bell contraction and relaxation. We use 3D PTV (shake-the-box) technique to examine the currents generated by pulsing \textit{Cassiopea} medusae with and without oral arms. Our results show that it is possible to generate unidirectional flow in the vertical direction even without oral arms. The importance of considering three-dimensionality in complementary modelling efforts will be discussed.