Although prey capture by cnidarians is mediated through nematocysts, their influence on prey selection by cnidarians remains poorly documented. The difficulty in visualizing nematocyst–prey interactions remains the chief obstacle to understanding how the wide variety of nematocyst types influences the mechanics of prey capture. One solution to this limitation has been to assign functional roles to nematocysts based on morphological characters of discharged cnidae. Here we report results of an alternative approach based upon dynamic traits of nematocyst discharge. We examined tubule lengths, tubule discharge velocities and net-to-gross displacement ratios of tubules of discharging nematocysts possessed by the cosmopolitan scyphomedusa, Cyanea capillata. This nematocyst assemblage consisted of euryteles, birhopaloids and three different isorhizas — a-isorhizas, A-isorhizas and O-isorhizas. Dynamic traits varied little within each nematocyst type but there were significant differences between the different types. Most importantly, dynamic traits varied significantly within a broad category of nematocyst – the isorhizas – indicating that conventional classification schemes that infer function based on broad nematocyst categories may not appropriately describe the functional roles of these nematocysts. The dynamic properties of discharging nematocysts were consistent with physical results described in studies using scanning electron microscopy images of nematocyst–prey interactions. These data suggest that nematocysts vary significantly in their roles during predation, but that inferences relating prey selection with broad nematocyst categories merit careful examination.