Abstract:  As organisms swim in their natural environment, they are constantly striving to successfully forage, escape from predators, and search for mates to reproduce. Marine organisms are accomplishing these tasks while interacting with their fluid environment, generating hydrodynamic signatures in their wake and around their bodies. The resulting hydrodynamic signatures are detected by predators and pray, and are also indicative of energetics and proficiency of swimming organisms. Changes in hydrodynamic signatures as an organism grows and develops provide useful insight into how an organism interacts and perceives its fluid environment. A widespread swimming strategy employed by squid, salps, and medusae is jet propulsion. Although jet propulsion in medusa has been shown to be energetically costly, organisms remain motionless most of the time and forage as ambush predators. In this study, we evaluate swimming by hydromedusae Sarsia tubulosa from viscous to inertial flow regimes (1 mm to 1 cm bell diameters) using high-speed video recordings and digital particle image velocimetry. We employ biomechanical metrics (proficiency and swimming efficiency) and quantify vortex formation characteristics (dimensionless energy α and formation time) to accurately compare swimming ability as S. tubulosa grows. Our results show that jet propulsion becomes less proficient and less efficient with bell diameter, which has important implications for larger organisms using jet propulsion. 

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Host:  Eckart Meiburg