Immediate and delayed life-history responses of Daphnia ambigua to conspecific cues

Studies of planktonic organisms have shown that various life-history traits, and consequently population growth rate, can be affected by chemical cues released by high densities of conspecifics. In this work we test for potential delays in population responses to conspecific cues in the cladoceran Daphnia ambigua through: (i) evaluating the influence of maternal environment on life-history traits of their daughters, and (ii) identifying the developmental stages that need to be exposed to conspecific cues to observe significant changes in life-history attributes. Our results revealed that exposure to conspecific cues decreased population growth rate of D. ambigua, mainly due to reduced offspring number and delayed maturation. Growth rate reduction was also dependent on the mother's environment, and interaction effects arose between current generation and past-generation environments. The observed variations in maturity and fertility were triggered by exposure to conspecific cues during early development. We hypothesize that conspecific cues are able to induce delayed life-history effects and therefore delayed density dependence at the population level.