MORPHOLOGY, FLUID MOTION AND PREDATION BY THE SCYPHOMEDUSA AURELIA-AURITA

Although medusan predators play demon strably important roles in a var iety of marine ecosystems, the mechanics of prey capture and, hence, p rey selection, have remained poorly defined. A review of the literatur e describing the commonly studied medusa Aurelia aurita (Linnaeus 1758 ) reveals no distinct patterns of prey selectivity and suggests that A . aurita is a generalist and feeds unselectively upon available zoopla nkton. We examined the mechanics of prey capture by A. aurita using vi deo methods to record body and fluid motions. Medusae were collected b etween February and June in 1990 and 1991 from Woods Hole, Massachuset ts and Narragansett Bay, Rhode Island, USA. Tentaculate A. aurita crea te fluid motions during swimming which entrain prey and bring them int o contact with tentacles. We suggest that this mechanism dominates pre y selection by A. aurita. In this case, we predict that medusae of a s pecific diameter will positively select prey with escape speeds slower than the flow velocities at their bell margins. Negatively selected p rey escape faster than the medusan flow velocity draws them to capture surfaces. Faster prey will be captured by larger medusae because flow field velocity is a function of bell diameter. On the basis of prey e scape velocities and flow field velocities of A. aurita with diameters of 0.8 to 7.1 cm, we predict that A. aurita will select zooplankton s uch as barnacle nauplii and some slow swimming hydromedusae, while fas ter copepods will be negatively selected.