Nutrient-specific foraging by Glossosoma intermedium larvae leads to conspecific case grazing

Nutrient-specific foraging, or selection of resources based on nutritional quality, is a strategy homeo-static consumers use to meet their dietary nutritional needs. Glossosoma intermedium is a homeostatic caddisfly that often grazes nutrient-rich periphyton colonizing the cases of conspecific larvae. We hypothesized that case grazing may be the result of nutrient-specific foraging under conditions where streambed periphyton is nutrient deficient. To test this hypothesis, we monitored larvae in experimental streams. We measured case-grazing frequency and duration under 4 treatments of streambed periphyton: ambient (control), +N, +P, and +NP. Casegrazing frequency was significantly lower in the +NP treatment than in the other treatments, suggesting that nutritional quality influences resource selection by G. intermedium. However, duration of case-grazing bouts was not influenced by treatment. In addition, grazing by G. intermedium increased streambed periphyton C: P and N:P more in the +P treatment than in the other treatments, whereas grazing did not affect streambed periphyton C: N or algal biomass differently among treatments. This result suggests that Glossosoma was P limited, and that it either differentially ingested P-rich periphyton or differentially retained P. Our study suggests that nutrient imbalances between streambed periphyton and grazer nutrient demand lead to case grazing by G. intermedium larvae and that the different strategies (case grazing, selective foraging, differential nutrient excretion/retention) that P-limited grazers use to meet their nutritional needs lead to P depletion of the surrounding periphyton mat.