Effect of prey size on vulnerability of copepods to predation by the scyphomedusae Aurelia aurita and Cyanea sp.

A growing number of studies correlate changes in zooplankton populations with abundance of medusae, but we cannot yet explain or predict the specific factors driving these interactions. This study demonstrates that the size of copepods has a significant influence on their vulnerability to predation by scyphomedusae. This finding is important because prey size, independent of behavior, has been neglected in theoretical models of predation by medusae. In experiments in a planktonkreisel, we used live and heat-killed prey (Acartia hudsonica adults and copepodites) to separate the effects of copepod size and behavior on feeding rates by two medusae (Aurelia aurita and Cyanea sp.). Results revealed that: differences in copepod size had a significant impact on feeding rates, and thus small size can provide a refuge from predation; behavior of adults diminished the liability associated with larger size; and medusae with different morphologies ingested A.hudsonica at similar rates. Other experiments demonstrated that medusae digested copepods at different rates based on prey size and predator species, findings that have implications for all future laboratory and field studies that assess feeding by scyphomedusae. Finally, this study illustrates how laboratory studies serve as critical supplements to field observations. The effect of prey size on feeding rates can be confounded by differences in prey behavior, yet explains why small copepods were typically ingested at relatively low rates by medusae. Size was clearly a dominant factor influencing copepod vulnerability across scyphomedusan species, even those with very different morphologies. Future work should focus on the mechanisms of size selection, or the factors influencing contact and retention rates.