MECHANICS OF PREY SELECTION BY EPHYRAE OF THE SCYPHOMEDUSA AURELIA-AURITA

In situ feeding patterns of ephyrae of the jellyfish Aurelia aurita (L innaeus) revealed the importance of relatively large (>1 mm) prey in t he diet of these scyphozoan predators. These studies were carried out in Narragansett Bay, Rhode Island, USA in March and April, from 1993 t hrough 1996. Rotifers were the only small prey ingested in quantity, a nd then only when they were unusually abundant in the plankton. Copepo d nauplii, similar in size to rotifers and equally abundant, were rare ly consumed. Since copepods evince rapid escape responses, this observ ation suggested a role for prey escape in determining prey vulnerabili ty, while the predominance of large prey in the diet suggested a role for prey size. Using two dimensional video observations of free-swimmi ng ephyrae and their prey in the laboratory we tested hypotheses about the mechanisms underlying these dietary patterns, comparing mechanism s for capture of large versus small prey and for prey of equal size bu t differing escape behaviors. Capture efficiencies of ephyrae feeding on large prey were 4 to 12 times greater than for small prey taxa. Cap ture efficiencies for prey of equal size also differed significantly, indicating that other factors influence the outcome of predator-prey i nteractions. Most prey captures occurred while the ephyrae were swimmi ng and creating fluid flows that entrained prey into the subumbrellar region. Even copepod nauplii were frequently drawn into the subumbrell a of swimming ephyrae despite average potential escape velocities (25. 7 mm s(-1)) that exceeded mean maximum velocity of fluid flows around the ephyrae (13.1 mm s(-1)). Large prey were more likely than small pr ey to contact nematocyst-bearing surfaces both before and after entrai nment in flow fields. With regard to behavior, prey escape speeds were not the only predictor of prey vulnerability. Prey that continued swi mming after entrainment (rotifers and brine shrimp) were captured more often than prey of equal size that ceased normal swimming (copepod na uplii and barnacle nauplii). Copepod nauplii were the prey least likel y to be captured because they either ''played dead'' and were expelled from the subumbrella of the ephyrae before contacting a surface, or t hey eventually escaped at high velocity. These observations indicate t hat size-selective predation by ephyrae of A. aurita can be influenced by a variety of behavioral responses of the prey.