## 4 Discussion
Consistently higher temperatures or oxygen availability are possible explanations for the increase in MMR, which caused the change in AS, at DS1 and DS2 in RBDs and FTDs (Table 1). The DO and temperature may also vary in the microhabitats the darters live in, permitting an increase in oxygen availability [57; 58]. [PERSON] et al. demonstrated that in spiny damselfish, _Acanthochromis polyacanthus_, AS increased in fish that were exposed to environmental perturbations, contrary to the expected decrease when faced with an environmental challenge [59]. Sunfish (_Lepomis gibbosus_) can vary their hematocrit levels, altering their ability to transport oxygen to the tissues, which may explain how the fish downstream from the MWWTP are compensating to increase their MMR [60]. Fishes can also increase perfusion and recruitment of the gills to increase MMR [28]. Darters may be utilizing these strategies to increase metabolic capacities. Darters preferentially select water temperatures just above 20 \({}^{\circ}\)C, which may explain why there is an increase in their aerobic capacity in the summer over the fall [61]. Additionally, the food and habitat resources of darters overlap, causing intraspecific competition, specifically between RBDs and FTDs [62]. These two species are closely related and have similar prey; in the summer they may be similarly affected by ecological variability and therefore more sensitive to environmental upsets due to additional stressors [63]. Since GSDs do not face the same degree of competition, the metabolic adjustments may not be necessary. Alternatively, GSDs are more sensitive to changes in oxygen availability and temperature preferences, and therefore, have less ability to modify aerobic capacity [57; 64]. Regardless, the evidence presented here does suggest there were differential species-specific responses to living in MWWTP effluent contaminated sites. Damage due to contaminants can directly change the morphology of the gills, for example, epithelial lifting when toxic substances interact with the gills [65]. The most common pathologies associated with chronic, sublethal exposure to various xenobiotics are hyperplasia, edema, and cell proliferation [66]. The gills in this study demonstrated evidence of pathologies at all sites, but a significant increase in the presence and severity, as well as thickened lamellar epithelium, was found at the sites downstream from the MWWTP in RBDs before the upgrades. After the upgrades, there was no evidence of increased pathologies in RBDs compared to the reference site, which may indicate that the water quality has improved, which reduced the response of the gills to irritants in the water. Overall, the pathologies are thought to be defense mechanisms of the fish to limit the entry of irritants as a general stress response rather than a contaminant-specific reaction [30]. Thickening of the epithelium, hyperplasia, and epithelial lifting all increase the oxygen diffusion distance but may provide a barrier to entry for contaminants [43; 67]. Increased water temperatures and decreased water quality due to lower flow during the summer can cause an increase in pollutant toxicity, which can explain the response of the gill morphology [68; 69]. All three species have an epithelium that is already \"thick\" due to being benthic [28]. An increase in SLW due to hyperplasia causes gill fusion, which hinders oxygen uptake and decreases carbon dioxide offloading, making this maladaptive [70; 71]. Over time, fish can resist the effects of contaminants if they are at sublethal concentrations via physiological modifications, for example, varying perfusion of the gills, increased respiration, increasing detoxification processes, or stimulating the immune system [67; 69; 72]. Overall, there was evidence of pollution effects on the gill morphology of RBDs, and in one season, on GSDs, while FTDs did not show any maladaptive response.