Mechanics of prey selection by ephyrae of the scyphomedusa Aurelia aurita |
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Authors: | B K Sullivan C L Suchman J H Costello |
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Institution: | (1) Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, USA, IS;(2) Biology Department, Providence College, Providence, Rhode Island 02918, USA, IS |
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Abstract: | In situ feeding patterns of ephyrae of the jellyfish Aurelia aurita (Linnaeus) revealed the importance of relatively large (>1 mm) prey in the diet of these scyphozoan predators. These studies
were carried out in Narragansett Bay, Rhode Island, USA in March and April, from 1993 through 1996. Rotifers were the only
small prey ingested in quantity, and then only when they were unusually abundant in the plankton. Copepod nauplii, similar
in size to rotifers and equally abundant, were rarely consumed. Since copepods evince rapid escape responses, this observation
suggested a role for prey escape in determining prey vulnerability, while the predominance of large prey in the diet suggested
a role for prey size. Using two dimensional video observations of free-swimming ephyrae and their prey in the laboratory we
tested hypotheses about the mechanisms underlying these dietary patterns, comparing mechanisms for capture of large versus
small prey and for prey of equal size but differing escape behaviors. Capture efficiencies of ephyrae feeding on large prey
were 4 to 12 times greater than for small prey taxa. Capture efficiencies for prey of equal size also differed significantly,
indicating that other factors influence the outcome of predator–prey interactions. Most prey captures occurred while the ephyrae
were swimming and creating fluid flows that entrained prey into the subumbrellar region. Even copepod nauplii were frequently
drawn into the subumbrella 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 prey to contact nematocyst-bearing surfaces both before and after entrainment in
flow fields. With regard to behavior, prey escape speeds were not the only predictor of prey vulnerability. Prey that continued
swimming after entrainment (rotifers and brine shrimp) were captured more often than prey of equal size that ceased normal
swimming (copepod nauplii and barnacle nauplii). Copepod nauplii were the prey least likely to be captured because they either
“played dead” and were expelled from the subumbrella of the ephyrae before contacting a surface, or they eventually escaped
at high velocity. These observations indicate that size-selective predation by ephyrae of A. aurita can be influenced by a variety of behavioral responses of the prey.
Received: 9 April 1997 / Accepted: 5 September 1997 |
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