Impacts of temperature and turbidity on the gill physiology of darter species.

Fish gills are complex organs that have direct contact with the environment and perform numerous functions including gas exchange and ion regulation. Determining if gill morphometry can change under different environmental conditions to maintain and/or improve gas exchange and ion regulation is important for understanding if gill plasticity can improve survival with increasing environmental change. We assessed gill morphology (gas exchange and ion regulation metrics), hematocrit and gill Na+/K+ ATPase activity of wild-captured blackside darter (Percina maculata), greenside darter (Etheostoma blennioides), and johnny darter (Etheostoma nigrum) at two temperatures (10 and 25 °C) and turbidity levels (8 and 94 NTU). Samples were collected August and October 2020 in the Grand River to assess temperature differences, and August 2020 in the Thames River to assess turbidity differences. Significant effects of temperature and/or turbidity only impacted ionocyte number, lamellae width, and hematocrit. An increase in temperature decreased ionocyte number while an increase in turbidity increased lamellae width. Hematocrit had a species-specific response for both temperature and turbidity. Findings suggest that the three darter species have limited plasticity in gill morphology, with no observed compensatory changes in hematocrit or Na+/K+ ATPase activity to maintain homeostasis under the different environmental conditions.