Fluid Mechanics of Feeding by the Hydromedusa Clytia gregaria Determines the Trophic Niche of this Ubiquitous Coastal Predator

Feeding selectivity in gelatinous predators has the potential to mediate energy flow through planktonic food webs. In current-feeding cnidarian hydromedusae, swimming and predation are coupled such that swimming also brings prey into contact with the medusa’s feeding structures. Fluid mechanical disturbances may then initiate escape responses in rheotactic prey. The goals of this study were to determine: 1) if passive (drifting) and active (swimming) feeding behavior of Clytia gregaria result in differences in predation on co-occurring prey types and, 2) if the differences in fluid signals produced by each swimming behavior correlate with the type of prey captured. Videography was used to quantify the predation process of natural prey types. Passive and slow swimming prey such as invertebrate larvae were encountered and captured during both types of feeding behavior, whereas prey with active escape responses such as copepods were only captured during passive feeding. Flow visualizations using particle image velocimetry showed that the fluid shear values during passive feeding were below detection thresholds of copepods. Two other species of hydromedusae-- Aequorea victoria and Mitrocoma cellularia-- produced higher shear during current feeding than C. gregaria, which helps explain a lack of rheotactic prey in their guts. Fluid signals produced by tentaculate predators are a phenotypic feature that defines their trophic niches and are relevant to understand how fluid flow mediates predator-prey interactions in the ocean.