While swimming in their natural environment, marine organisms must successfully forage, escape from predation, and search for mates to reproduce. In the process, planktonic organisms interact with their fluid environment, generating fluid signatures around their body and in their downstream wake through ontogeny. In the early stages of their life cycle, marine organisms operate in environments where viscous effects dominate and govern physical processes. Ontogenetic propulsive transitions in swimming organisms often involve dramatic changes in morphology and swimming behavior. However, for organisms that do not undergo significant changes in morphology, swimming behavior, or propulsive mode, how is their swimming performance affected?
We investigated the ontogenetic propulsive transitions of the hydromedusa Sarsia tubulosa, which utilizes jet propulsion and possesses similar bell morphology throughout its life cycle. We used digital particle image velocimetry and high-speed imaging to measure the body kinematics, velocity fields, and wake structures induced by swimming S. tubulosa from 1 mm to 10 mm bell exit diameters. Our experimental observations revealed three distinct classes of hydrodynamic wakes: elongated vortex rings for 10<Re<30 (1 to 2 mm bell exit diameter), classical elliptical vortex rings for Re>30 (larger than 2 mm bell exit diameter), and in most instances where Re>100 (larger than 4 or 5 mm bell exit diameter), elliptical vortex rings (or leading vortex rings) were followed by trailing jets. The relative travel distance and propulsive efficiency remained unchanged throughout ontogeny, and the swimming proficiency and hydrodynamic cost of transport decreased nonlinearly.
