- Mount Allison University
The central mudminnow (Umbra limi (Kirtland, 1841)) is a continuous, facultative air-breathing freshwater fish found in swamps of central Canada and northeastern USA. The first goal of this field and laboratory-based study was to characterize the physicochemical conditions of mudminnow habitat during the summer. Our second goal was to determine the metabolic, stress response, and nitrogen excretion strategies of this fish following variations in water temperature, dissolved oxygen, external ammonia, and short-term periods of air exposure. We report profound diurnal fluctuations in water temperature (13–31 °C), dissolved oxygen (2%–159% air saturation), and ammonia levels (10–240 μmol·L−1) in habitat of central mudminnow measured on three dates at six different sites over 24 h. The central mudminnow does not induce urea synthesis as a mechanism of ammonia detoxification, either in response to emersion (6 or 20 h) or elevated external ammonia (10 mmol·L–1). Acute exposure to high temperature (~31 °C), aquatic hypoxia, or air resulted in significant increases in blood glucose and liver heat shock protein (Hsp) 70 and hypoxia also caused an increased reliance on anaerobic metabolism. This is the first description of the heat shock response in a facultative air-breathing fish following either hypoxia or air exposure. These metabolic and molecular responses are part of a strategy that allows the mudminnow to thrive in extremely variable freshwater environments.
Field measurements were conducted at six pond sites near the Queen's University Biological Station, Lake Opinicon (44°34'N, 76°19'W), Ontario, where the mudminnow was captured. The six sites had similar water depths (39-59 cm) and physical features (i.e., shallow water with muddy substrates). Measurements were taken every 3 h for a 24 h period on three separate days (29 June, 4 July, 9 July 2007). We averaged several measurements at each site and at each time point to ensure that the entire site was represented. We sampled water temperature and dissolved oxygen (percent saturation) with a portable meter (HQd field kit; Hach, Anachemia Science, Mississauga, Ontario, Canada) and water samples were collected and frozen (-20 °C) for later analysis of ammonia via the salicylate-hypochloride assay (Verdouw et al. 1978). We also measured water ammonia concentration at three different depths and in filtered mud (Table 1) from one of the six sites (Osprey Marsh) near the Biological Station. These measurements were taken on three different dates in 2006 (4, 7, and 11 July). The mud was filtered in Whatman #1 filter paper and the filtrate was assayed for ammonia as above.
Mudminnows (4.30 g ± 0.13) were captured in unbaited minnow traps and seine nets between 30 June and 13 July 2006 and 2007. Fish were transferred to coolers containing aerated water and transported to the laboratory within ~2 h. In the laboratory, fish were held in flow-through tanks supplied with aerated lake water (Lake Opinicon; 24 ± 1 °C, pH 7.8 ± 0.2). Fish were not fed and were used for experiments within 4 days of capture. All experiments were approved by Queen's University and the University of Guelph's Animal Care Committee under the guidance of the Canadian Council on Animal Care.
Five experiments were conducted: (1) acute exposure to a range of water temperatures 15-31 °C; (2) acute exposure to hypoxic water (1 h, 15%-20% air saturation, 24 °C); (3) 6 h air exposure at 24 °C; (4) 20 h air exposure at 24 °C, and (5) acute exposure to high external ammonia (1 h, 10 mmol x L-1 NH4 Cl, 24 °C). In experiments 1-3, we measured aerobic metabolic rate, glucose and lactate concentration, Hsp levels, and nitrogen excretion rates. In experiments 4 and 5, we measured only nitrogen excretion rates.