Adaptive growth reduction in response to fish kairomones allows mosquito larvae (Culex pipiens) to reduce predation risk

Phenotypic plasticity is predicted to evolve when subsequent generations are likely to experience alternating selection pressures; e.g., piscine predation on mosquitoes (Culex pipiens) varies strongly depending on habitat type. A prey-choice experiment (exp. 1) detected a predilection of common mosquito predators (sticklebacks, Gasterosteus aculeatus) for large-bodied mosquito larvae, suggesting that larvae could benefit from suppressing growth under predation risk, and experiment 2 confirmed reduced pupa size and weight when we exposed larvae to stickleback kairomones. In experiment 3, we measured adult (imago) size instead to test if altered larval growth-patterns affect adult life-history traits. We further asked how specific life-history responses are, and thus, also used kairomones from introduced Eastern mosquitofish (Gambusia holbrooki), and from algivorous, non-native catfish (Ancistrus sp.). Adult body mass was equally reduced in all three kairomone treatments, suggesting that a non-specific anti-predator response (e.g., reduced activity) results in reduced food uptake. However, imagines were distinctly smaller only in the stickleback treatment, pointing towards a specific, adaptive life-history shift in response to the presence of a coevolved predator: mosquito larvae appear to suppress growth when exposed to their native predator, which presumably reduces predation risk, but also affects body size after pupation. Our study suggests that (1) not all antipredator responses are necessarily predator-specific, and (2) fluctuation in the cost-benefit ratio of suppressing larval growth has selected for phenotypic plasticity in C. pipiens larval life histories. This implies costs associated with suppressed growth, for example, in the form of lower lifetime reproductive success.