Swimming behaviour of Daphnia clones: differentiation through predator infochemicals

Various characteristics of population and community dynamics of freshwater zooplankton have been studied in the past, in attempts to increase our knowledge of how freshwater ecosystems function. A major leap forward in disentangling ecosystem processes was the discovery of communication based on chemical cues (infochemicals) in the early 1980s. Infochemicals were found to affect many aspects of zooplankton ecology, such as life history, morphology, physiology and behavior [see reviews by (Harvell, 1990; Larsson and Dodson, 1993; Tollrian and Dodson, 1998; Boersma et al., 1999)]. Swimming behavior is an important factor in the predator–prey relationship (Gerritsen, 1980; Pastorok, 1980, 1981; Riessen et al., 1984) because it affects the probability and the frequency with which prey and potential predators encounter each other. Encounter is the central process in governing direct and indirect interactions between prey and predators. The mechanism behind behavioral strategies of both prey and predator becomes clear with the help of theoretical models. According to the empirical model of Gerritsen, the encounter rate of a predator with its prey is a function of prey density and the mean speed of both predator and prey (Gerritsen, 1980). The encounter rate depends on the encounter radius, density of prey, and swimming speeds of both predators and prey. The model predicts two optimal tactics for efficient predators: the ambush tactic (often found in invertebrates) and cruising (found in planktivorous fish). For both tactics, swimming speed is a key trait in determining vulnerability of prey to predators. Either an increase in predator speed or an increase in prey speed, or both, can increase encounter rate. The greater of the two speeds has the strongest effect. This is important for slow animals, because a further decrease in speed of a slow-moving prey would result in a negligible decrease of its encounter probability with a fast-moving predator. The same holds for the reversed situation with a slow-moving predator and fast-moving prey.