Meeting Abstract
Obelia sp. are cnidarian hydromedusae with a cosmopolitan distribution but we currently lack a mechanistic basis of their feeding behavior. The small size of Obelia (Bell diameter ~1 mm, tentacle width ~0.05 mm) suggests that feeding occurs in a viscous regime characterized by thick boundary layers. Swimming kinematics from high speed videography confirmed that swimming was a low Re number process (Re<50) and showed that maximum tentacle velocities occurred at the tentacle tips midway through a bell contraction. Flow visualizations from Particle Image Velocimetry demonstrated that fluid motion between the tentacles was limited and that velocity was highest at the tentacle tips, suggesting a thinning of the boundary layer. Feeding observations with a natural prey assemblage supported swimming kinematic and fluid motion measurements: the majority of prey were captured at the tentacle tips at the time when tentacle velocities were highest. Furthermore, nematocyst counts using differential interference contrast microscopy showed the highest numbers of nematocysts at the tentacle tips. Taken together, the body motion, fluid manipulation and nematocyst distribution of Obelia explain how these important predators are able to shed viscous boundary layers to effectively capture prey.
