Crayfish must locate food resources in a chemically complex environment in which chemicals from various sources interact and mix. We tested the hypothesis that an additional chemical food source of leachate from detritus (dissolved organic matter [DOM]) would affect crayfish orientation behavior toward a fish stimulus. In addition, we predicted that the effect of DOM would differ depending on whether the detritus was produced under ambient (AMB) or elevated (ELEV) CO2 because of changes in foliar chemistry that accompany elevated CO2 levels. DOM was collected from leaf litter derived from quaking aspen (Populus tremuloides) foliage that was produced at either AMB or ELEV CO2 and was presented at 2 concentrations (3 mg/L, 6 mg/L). Stimulus treatments were: 1) CON (control; fish odor only), 2) AMB-low (fish AMB DOM at 3 mg/L), 3) AMB-high (fish AMB DOM at 6 mg/L), 4) ELEV-low (fish ELEV DOM at 3 mg/L), and 5) ELEV-high (fish ELEV DOM at 6 mg/L). Crayfish (Orconectes virilis) were observed in a recirculating flume in which the fish odor source was placed downstream of the DOM odor source. Behavioral responses measured were % success in locating the fish odor source, time to find source, walking speed, walking speed toward the source, turning angle, heading angle relative to the source, heading angle relative to upstream, and net-to-gross ratio (NGR). Crayfish in the AMB-high treatment were most successful in locating the source. Animals had higher turning angles in the ELEV-high treatment than in all treatments except the control and had increased heading angles relative to the source and decreased heading angles relative to upstream. No differences were found for walking speed or NGR. These results indicate that crayfish orientation to a fish odor source is affected by the presence of DOM from detritus, but only when it is presented at the high end of a natural range of DOM concentration. It also appears that the presence of ambient CO2 detritus (with a lower concentration of secondary defensive compounds) increases success in finding the source, whereas the presence of elevated CO2 detritus does not. Possibly, in an elevated-CO2 future, behaviors associated with foraging and selection of food types by crayfish might be altered, and therefore affect the aquatic community. These effects might be seasonal and synchronized with the natural fluctuations of DOM found in lotic systems.
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