Parasitic infection may affect the feeding rate of hosts and thereby directly or indirectly impact community structure. Infected hosts may increase feeding rate to offset the cost of infection or feeding rate may decrease because of host morbidity. The direction and strength of these interactions are difficult to predict so we empirically derived the functional response of mottled sculpins (Cottus bairdii) that were healthy or infected by Glugea sp. (Microsporidia). Fish were collected from Spring Brook (an uninfected stream), Kalamazoo County, MI, USA, measured for total length and weighed at the start of the experiment and then randomly assigned to exposure or control groups. Glugea exposed fish were orally administered 0.1 ml (1 x 105 spores/ml) spore solution and control fish were treated the same but given distilled water. We conducted three replicates at six prey densities for each disease state (2 disease states x 6 prey densities x 3 replicates) at four and ten weeks post infection for a total of 72 trials. An additional 40 trials were conducted at constant prey density (2 disease states x 20 replicates). All trials were run in 57L recirculating streams tanks over 24 hours using Gammrus as the prey.
Our results indicate that sculpins have a type II functional response regardless of infection status. However, moderate to heavily infected individuals (parasite mass ≥ 2% of host mass) have a significantly reduced feeding rate compared to healthy fish. Field surveys indicate that 10-25% of infections are moderate to heavy in natural sculpin populations. Together, these results suggest that Glugea infection in sculpins may affect benthic community structure in field populations.