Morphological and life-history shifts of the exotic cladoceran Daphnia exilis in response to predation risk and food availability

The zooplankter Daphnia exilis was found recently in reservoirs of central Chile. This species has been described as being tolerant to osmotic stress and chemical pollution, although the available information about its ecological properties is limited. Motivated by likelihood of being witnesses to a biological invasion, in this study we contribute to expand the knowledge of this exotic species through evaluating its ability to respond phenotypically to two major ecological factors: predation risk and food availability. Specifically, we analysed shifts in life-history and morphological traits of D. exilis in response to fish-released infochemicals, at different food densities. Our results revealed that the organisms were affected in their temporal trajectories of body size and shape, as well as in maturation time and fertility, in response to both predator cues and food availability. The presence of fish kairomones led to a decrease of age at maturity, and an increase of reproduction size and fecundity, especially at lower levels of resources. Our analyses indicated that asymptotic body size was affected only by food level, but the rate of increase in body size was sensitive to both fish kairomones and food level. The relative length of the tail spine decreased during early ontogenetic states, reaching a minimum around the age at maturity. This pattern was significantly enhanced in the presence of fish kairomones. However, our results did not match completely the typical responses of daphnids to fish kairomones.