Differential Location of Chaoborus Larvae and Daphnia by Fish: The Importance of Motion and Visible Size

Abstract

The at which white crappie (Pomoxis annularis) could locate. Chaoborus larvae (C. americanus and C. punctipennis) and Daphnia magna was measured. This distance increased as a linear function of prey size for both prey types. Prey motion more than doubled the maximum at which Chaoborus larvae could be located. Significant reactive changes also occurred as the orientation of Chaoborus to the predator varied. The reactive to body-length relations of Chaoborus and D. magna differed and indicated that Chaoborus were less visible than Daphnia. INTRODUCTION The importance of understanding the impact of fish predation on zooplankton community structure is widely accepted (O'Brien, 1979). Due to the fact that most planktivorous fish locate their prey visually, characteristics that influence zooplankton visibility are a central component in determining predator selectivity (Zaret, 1975). The factors that affect selectivity may be studied by measuring the at which a fish behaviorally demonstrates a recognition of a specific prey type, the reactive distance. The reactive of a number of fish species to a variety of prey types has been investigated (Ware, 1973; Confer and Blades, 1975; Werner and Hall, 1974; Vinyard and O'Brien, 1976; Kettle and O'Brien, 1978). These studies indicate that three factors must be considered: the visual acuity of the planktivore, the specific anatomical and behavioral characteristics of the prey and the physical characteristicsKof the environment. This study is concerned with the anatomical and behavioral traits of Chaoborus larvae (C. americanus and C. punctipennis) and Daphnia magna that elicit location and attack by a common planktivore, the white crappie (Pomoxis annularis). The initial hypothesis put forward by Brooks and Dodson (1965) and Hrbaicek (1962), that planktivorous fish predation is size-selective, has been modified by recent investigators. The prey's visible size to the predator is now considered to be the critical parameter, not its absolute size. For some prey species, the dimensions of the eyes appear to define the visible size (Zaret, 1972), while for others, such as Daplnia magna, the core body size is important (Vinyard and O'Brien, 1975; Confer et al., 1978; Kettle and O'Brien, 1978). Chaoborus, or phantom midge, larvae are virtually transparent except for the eyes and dark pigmentation over the two hydrostatic organs. These highcontrast structures are expected to be critical in defining the visible size of Chaoborus to a predator. The elongated cylindrical shape of Chaoborus also suggests that the portion of its body visible to a predator should vary with predator-prey orientation. Motion is another component of prey visibility. Ware (1973) showed that motion increased the at which inanimate food items could be located by rainbow trout. Motion or its absence might be particularly important in the detection of Chaoborus spp., which are, at times, an extremely important component of the white crappie's diet (Mitchell, 1941; Keast, 1968; Ball, 1972). Chaoborus are ambush predators which hang motionless in the water and tactually sense the approach of zooplankton prey. Prey capture requires a darting movement towards the source of the