Upstream foraging by medusae

Convergence upon similar morphological traits is common among predators feeding on similar prey in similar environments. Aboral placement of tentacles during swimming appears to be such a trait and has evolved in parallel among several medusan lineages. Here we examine this trait in 1 scyphomedusa and 2 hydromedusae representing different lineages within the Medusozoa with the goal of evaluating whether aborally directed tentacles function similarly among these varied medusae. We analyzed flow generation, swimming kinematics and swimming behavior of Craspedacusta sowerbyi (Hydrozoa, Limnomedusa), Nausithoee punctata (Scyphozoa, Coronata), Solmissus albescens (Hydrozoa, Narcomedusa) and the prey capture mechanics of N. puncatata. The 3 species swam similarly and generated similar wake structures in comparison to each other and to oblate medusae with trailing tentacles. Foraging behavior, as measured by the percent of time spent swimming, was similar for medusae possessing both leading and trailing tentacle placement. These findings indicate that medusae possessing either type of tentacle placement swim and forage similarly but that medusae with leading tentacles capture prey 'upstream' while medusae with trailing tentacles capture prey 'downstream' of vortices generated at the bell margin during swimming. These differences in tentacle placement relative to flow around the medusae may have important consequences for prey capture and trophic interactions that lead to different prey-selection patterns.