In situ time budgets of the scyphomedusae Aurelia aurita, Cyanea sp., and Chrysaora quinquecirrha

The in situ behavior of three scyphomedusan species was video recorded by scuba divers in natural daytime lighting with minimal interference to the medusae. The mean percentage of time that individual medusae spent swimming ranged from 93 to 100%. There were no significant differences in the percent time spent swimming between life stages of a species (ephyra, adult) or between species. The predominance of swimming activity by medusae indicates that swimming, and hence the creation of fluid motions responsible for prey entrainment and capture, plays a widespread functional role in feeding by scyphomedusae. Scyphomedusae have been found to be selective predators (Fancett, 1988; Fancett and Jenkins, 1988; Purcell, 1992; Sullivan et al., 1994) which have important effects on marine planktonic communities (Feigenbaum and Kelly, 1984; Moller, 1984; Purcell, 1992; Cowan and Houde, 1993; Purcell et al., 1994). Prey selection by scyphomedusae depends on the mechanisms of prey capture and the behavioral responses of prey (Costello and Colin, 1994,1995; Ford et al., 1997). The first of these, prey capture mechanics, depends on the foraging method used by the medusa. Whereas many planktonic cnidaria sit relatively motionless until prey swim into the medusa's outstretched tentacles (i.e. ambush entangling predators as described by Madin, 1988; Mills, 1981; Purcell, 1981,1984; Greene, 1985), many scyphomedusae create feeding currents via bell pulsation during swimming (Larson, 1991; Costello and Colin, 1994, 1995; Ford et al., 1997). However, the relevance of these laboratory studies of scyphomedusan feeding currents to actual field conditions has remained undemonstrated because no reports have quantified whether scyphomedusae spend significant proportions of time swimming and, hence, creating feeding currents under natural conditions. The goal of this study was to record in situ time budgets for representatives of three cosmopolitan scyphomedusan genera: Aurelia, Cyanea and Chrysaora. The relationships between swimming, fluid motion and prey capture have been described based on laboratory studies of each species (Aurelia aurita, Costello and Colin, 1994; Cyanea capillata, Costello and Colin, 1995; Chrysaora quinquedrrha. Ford et al, 1997). Our goal was to assess in situ swimming behavior, particularly the proportions of time allocated to swimming, by these medusae. In situ data were hand collected by scuba divers in natural daytime lighting. Medusae were videotaped on Hi8 videotape using a Sony TR81 camera with a zoom lens contained within an Amphibico underwater housing. Camera © Oxford University Press 383 D ow naded rom http/academ ic.p.com /plankt/articleact/20/2/383/1598896 by gest on 15 M arch 2020 J.H. CosteUo, E.Klos and M.D.Ford orientation paralleled the sediment and water surface (Figure 1). All dive sites were shallow, <7 m in depth, so that surface:bottom orientation was readily maintained. Aurelia aurita were sampled in Greenich Cove, RI, USA, whereas Cyanea sp. and Gquinquecirrha were sampled in Niantic River, CT, USA. Disturbance of medusae during videotaping was minimized by facing either perpendicularly into the current or parallel to the direction of the current. By doing so, medusae were carried by ambient tidal currents past the video camera and thus experienced no mechanical disturbance from diver motions until actually passing the diver. Video used for time budget determinations included only footage from periods during which the medusae were undisturbed by divers. Thus, lighting and mechanical disturbance were negligible factors influencing medusan behavior. During periods of slack tide or minimal current, medusae were videotaped from as great a distance as water clarity and magnification limits would allow. This ranged from several centimeters for ephyrae to over a meter for larger adult medusae. Medusan behavior was divided into two categories: swimming (active bell pulsation) and resting (no bell pulsation). The time course of medusan behavior was compiled using frame-by-frame examination of time code-labeled videotapes (Costello etai, 1990). Swimming direction was measured with reference to an angular grid having a central, vertical axis running from the sediment (270°) to the water surface (90°; Fig. L In situ video recording of medusa swimming. The camera viewing axis paralleled the sediment and water surface. Medusa orientation was described relative to a polar coordinate system as illustrated. When present, current was arbitrarily set as approaching from 0°. 384 D ow naded rom http/academ ic.p.com /plankt/articleact/20/2/383/1598896 by gest on 15 M arch 2020 In situ time budgets of scyphomedusae see Figure 1). By this convention, medusae swimming upwards, perpendicular to the planes determined by the bottom and water surface, were assigned an angular value of 90°. Likewise, medusae swimming downward were assigned a value of 270°. The other primary axis, running from 0 to 180°, was determined with reference to ambient tidal flow. Directional measurements were based on two-dimensional video images. In order to minimize bias due to motion in the unmeasured third dimension, angular direction measurements were limited to medusae for which both the front and back bell margins were level and, therefore, moving in the two-dimensional viewing plane. By this criterion, the number of medusae on any sample date that were suitable for angular direction measurements was typically less than for time budgets on that date. Angular direction measurements were compiled by placing a polar coordinate template over the video monitor and estimating angular motion in 5° increments. These measurements were taken at 30 s intervals for each medusa; for extended observation times of individual medusae, the measurements compiled over successive 30 s intervals were averaged. Preliminary analysis of time budget data demonstrated that most medusae swam continuously and that the percent of time spent swimming per medusa was not normally distributed (Shapiro-Wilks, P < 0.001), that variances were not homogeneous between groups (Bartlett, P < 0.01) and that standard deviations