Alternative antipredator responses of two coexisting Daphnia species to negative size selection by YOY perch

Abstract

Our study showed for the first time in nature that two coexisting Daphnia adopted alternative life history and behavioural strategies to cope with negative size-selection predation by gape-limited young-of-the-year (YOY) perch. We evaluated the phenotypic plasticity in life history and behavioural traits of two coexisting Daphnia species, D. pulicaria (2 mm) and D. galeata mendotae (1.4 mm), in response to seasonal changes in predation by YOY yellow perch (Perca flavescens) in a mesotrophic lake. We expected that the large-sized D. pulicaria, the most likely subjected to sizeselective predation by YOY perch, will show stronger antipredator responses than the small-sized D. galeata mendotae. To test this hypothesis, we examined changes in life history and behavioural traits in juveniles and adults of both species during four YOY fish predation periods that were selected based on the presence of YOY perch in the pelagic zone and the relative abundance of Daphnia prey in their gut contents. Our study supports the scenario of negative size-selective predation by gape-limited YOY perch on both Daphnia species. The electivity index indicated that no daphnids with a body length .1.75 mm were predated by YOY yellow perch. Coexisting Daphnia exhibited phenotypic plasticity in their antipredator defenses based on their vulnerability to seasonal changes in size-selective predation of YOY perch. Juvenile Daphnia were the targeted prey and they responded by a decreased body length. Behavioural defenses were the dominant strategy used by both adult Daphnia populations to withstand high predation. A decreased size at maturity was not employed by Daphnia, except at the very end of the predation period. Behavioural defenses are short-term strategy adopted to avoid predation. Both antipredator defenses became unnecessary expenses and were no longer sustained after the predation period.