Ref ID : 969
Steven W. Hewett; Predation by Didinium nasutum: Effects of predator and prey size. Ecology 69(1):135-145, 1988
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Experimental manipulation of the size of Didinium nasutum, a protozoan predator, feeding on large and small Paramecium species was used to examine the effects of predator and prey size on predator behavior and capture success. Didinium size is a function of prey size; large and small Didinium were obtained by conditioning predators on large or small prey species prior to experimentation. The predatory behavior of large and small predators feeding on large and small prey was monitored through an entire division cycle. Large Didinium had shorter handling times and encounter times on both sizes of prey than small Didinium. Large Didinium made more captures per division on either size of prey. Large prey required longer handling times, longer engulfing times, and were encountered more frequently than small prey for both sizes predator. First captures were more difficult than subsequent captures for all predators. Small predators were more successful initially in capturing small prey than were large predators; first captures by large predators on small prey were particularly difficult. Encounter times, the number of encounters per capture times, search times, and handling times varied with the size of the predator and also varied within the division cycle of the predator. The effect of time within the division cycle may be due to increasing size as prey are captured and/or to adaptations to the specific prey species following initial captures. Didinium may adapt to new prey species through morphological changes in the oral apparatus, chemoreceptors, or extrusive organelles, in addition to shifts in size. That each size of Didinium had more successes per encounter on the size of prey to which it had been conditioned suggests that the size shift improves that aspect of the predatory behavior. However, other aspects (such as handling time and time to each encounter) show a clear advantage to large size. Large size also improves survival ability of Didinium under conditions of starvation (Hewett, 1987). For large predators to maintain a large size on small prey would require a substantial increase in the division time. Selection forces favoring fast division rates, may impose restraints, making the size of Didinium on any prey species a compromise.