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Understanding the sublethal effects of pesticides is critical because most pesticides are found in low concentrations in nature. Sublethal effects (i.e. effects that do harm to organisms, but do not kill them) have been understudied. I am exploring the sublethal effects of malathion on life history traits of Hyalella amphipods. Amphipods collected from two populations in western Oklahoma will be exposed to one of three concentrations of malathion: a no malathion control (0 pg/L), a relatively low sublethal concentration (0.005 pg/L), and a relatively high sublethal concentration (0.02 pg/L). Amphipods that have just reached maturity will be chosen for the experiment and monitored for the entirety of their adult lifespan. For both sexes, I will measure growth rate. For each female, I will also record the number of offspring produced over her lifetime to measure lifetime reproductive success. For males, I will also measure the growth of the posterior gnathopod (a claw-like appendage), which is a sexually selected trait. I predict that the amphipods in the high concentration treatment will have slower growth rates, lower lifetime reproductive success, and smaller claws than those exposed to lower concentrations of malathion. If malathion negatively affects amphipod life history traits, there may be problems in the community they live in because they play important roles as grazers, detritivores, and prey in freshwater ecosystems.