Transitions in morphologies, fluid regimes, and feeding mechanisms during development of the medusa Lychnorhiza lucerna

Abstract

The early ontogeny of scyphomedusae involves morphological and functional transitions in body plans that affect the predators’ propulsive and feeding strategies. We applied high-speed videography, digital particle image velocimetry (DPIV) and dye visualization techniques to evaluate alterations in swimming and feeding mechanisms during ontogeny of the rhizostome medusa Lychnorhiza lucerna Haeckel, 1880 (Scyphozoa, Rhizostomeae). During early ontogeny, the ephyral mouth lips develop into complex filtering structures along the oral arms. The viscous environments (Reynolds number <100) experienced by ephyrae constrain the feeding mechanisms that transport fluid during ephyral bell pulsations. In contrast, adult medusan fluid flows are dominated by inertial forces and bell pulsations effectively transport fluids and entrained prey toward the oral arms. The oral arm surfaces are covered by motile epidermal cilia that drive these entrained flows through filtering gaps in the oral arms where food particles are retained. In addition to this process within the oral arms, vortices generated during bell pulsation flow downstream along the outside of the medusae and continuously transport prey toward the exterior oral arm surfaces. Although calanoid copepods are capable of escape velocities that greatly exceed L. lucerna’s feeding current speeds, copepods often fail to detect the predator’s feeding currents or inadvertently jump into medusan capture surfaces during failed escape attempts. Consequently, the comparatively weak predator feeding currents successfully capture a portion of the copepods encountered by swimming medusae. These results clarify the processes that enable rhizostome medusae to play key roles as consumers in tropical and subtropical coastal environments.