For large mobile organisms (>1 cm) the fluid environment (e.g., air or water) varies over time and across space. This is particularly well-documented for organisms living in freshwater aquatic systems. Streams and rivers serve as a useful example since they both show highly variable discharges. During periods of high precipitation these ecosystems can increase in discharge by more than 100 times, greatly altering the flow environment experienced by the organisms that call these habitats home. Not all individuals of a single species live in one habitat. We have documented the same species of crayfish in both streams and lakes. These animals experience unique chemosensory conditions created by the absence of constant flow in lakes and directional flow in streams. Thus organisms in these habitats experience unique sensory environments structured by the flow characteristics that are defined each habitat. Since selection favors those individual crayfish better suited to extract and respond to chemosensory information, my colleagues and I hypothesize that water flow conditions will control the chemical sensors and the chemically-mediated search behavior of animals. To investigate the validity of this hypothesis we will be conducting measurements of chemical signal dynamics in lake and stream environments. We will also conduct experiments to compare how lake and stream animals respond to chemical signals in their native and non-native habitat conditions.