Effects of temperature on acid-base regulation, gill ventilation and air breathing in the clown knifefish, Chitala ornata

Development and sex affect respiratory responses to temperature and dissolved oxygen in the air-breathing fishes Betta splendens and Trichopodus trichopterus

Ventilation frequencies of the gills (fG) and the air-breathing organ (fABO) were measured in juveniles and adults of the air-breathing betta (Betta splendens) and the blue gourami (Trichopodus trichopterus) in response to temperature and hypoxia. Ventilatory rates were evaluated after 1 h of exposure to 27 °C (control), 23 and 31 °C (PO2 = 21.0 kPa), after acute temperature changes (ATC) from 23 to 27, and 27 to 31 °C, and under progressive hypoxia (PH; PO2 =  ~ 21 to 2.5 kPa). Complex, multi-phased ventilatory alterations were evident across species and experimental groups revealing different stress responses and shock reactions (e.g., changes in temperature sensitivity (Q10) of fG between 1-h exposure and ACT in both species). Female and male gourami showed differences in Q10 over the temperature range 23-31 °C. No such Q10 differences occurred in betta. Juveniles of both species showed higher Q10 for fABO (~ 3.7) than fG (~ 2.2). Adult fish exhibited variable Q10s for fG (~ 1.5 to ~ 4.3) and fABO (~ 0.8 to ~ 15.5) as a function of temperature, suggesting a switch from aquatic towards aerial ventilation in response to thermal stress. During PH, juveniles from both species showed higher fG than adults at all oxygen levels. Females from both species showed higher fG compared with males. Collectively, our results suggest that environmental cues modulate ventilatory responses in both species throughout ontogeny, but the actual responses reflect species-specific differences in natural habitat and ecology. Finally, we strongly suggest assessing physiological differences between male and female fish to avoid masking relevant findings and to facilitate results interpretation.

Acute freshwater CO2 exposure does not impair seawater transfer in three different sizes of Atlantic salmon (Salmo salar) subjected to different photoperiod manipulations

There is a growing interest in Atlantic salmon (Salmo salar) aquaculture to extend the time fish are reared in freshwater (FW) recirculating aquaculture systems (RAS), producing larger FW salmon that can then be induced to undergo smoltification before transfer into marine net pens for grow-out and harvest. Smolts can be produced by photoperiod (PT) manipulation in RASs, but little is known about how delaying smoltification to larger body sizes affects susceptibility to elevated CO2 levels (hypercapnia), which can occur at high stocking densities in FW RAS or during transport from FW RAS rearing facilities to marine net pens. To address this, Atlantic salmon were reared from hatch to one of three different sizes (~230, ~580, or ~1300 g) in FW (3 ppt) under continuous light (24:0, light:dark). Once fish reached the desired sizes, a group of salmon were maintained on continuous light 24L:0D to serve as a control salmon. A second group of salmon were exposed to 8 weeks of 12L:12D and then to 4 weeks of 24L:0D to serve as PT treatment salmon, which is the PT manipulation commonly used in Atlantic salmon aquaculture to induce smoltification. At the end of PT manipulation, both control and PT treatment salmon were exposed to 0% or 1.5% CO2 (30 mg/L) for 96 h in FW and then transferred to air-equilibrated seawater (SW, 35 ppt, normocapnia). Salmon were sampled at the end of the 96-h FW CO2 exposure and at 24 h and 7 days in SW for measurements of blood ion/acid-base status, muscle water content (MWC), and gill and kidney Na+/K+ ATPase (NKA) activity. Exposure to 96 h of CO2 in FW resulted in acid-base disturbances in fish from all three size classes, with decreases in blood pH and increases in blood PCO2 and plasma [HCO3 -] but no mortality. Despite these large acid-base disturbances in FW, after transfer to normocapnic SW, there were no significant effects of CO2 exposure on extracellular blood pH, intracellular red blood cell pH, or plasma osmoregulatory status for all three sizes of post-smolt salmon. In general, SW transfer was associated with significant increases in plasma ions and osmolality, as well as gill and kidney NKA activity after 24 h and 1 week in SW with no significant impacts between different sizes of salmon. Thus, exposure to 30 mg CO2/L that mimics levels experienced during transport from FW RAS to an SW transfer site may have minimal effects on Atlantic salmon smolts up to 1300 g.

Modulation of gill surface area does not correlate with oxygen loss in Chitala ornata

Air-breathing fish risk losing aerially sourced oxygen to ambient hypoxic water since oxygenated blood from the air-breathing organ returns through the heart to the branchial basket before distribution. This loss is thought to help drive the evolutionary reduction in gill size with the advent of air-breathing. In many teleost fish, gill size is known to be highly plastic by modulation of their anatomic diffusion factor (ADF) with inter-lamellar cell mass (ILCM). In the anoxia-tolerant crucian carp, ILCM recedes with hypoxia but regrows in anoxia. The air-breathing teleost Chitala ornata has been shown to increase gill ADF from normoxic to mildly hypoxic water by reducing ILCM. Here, we test the hypothesis that ADF is modulated to minimize oxygen loss in severe aquatic hypoxia by measuring ADF, gas-exchange, and by using computed tomography scans to reveal possible trans-branchial shunt vessels. Contrary to our hypothesis, ADF does not modulate to prevent oxygen loss and despite no evident trans-branchial shunting, C. ornata loses only 3% of its aerially sourced O2 while still excreting 79% of its CO2 production to the severely hypoxic water. We propose this is achieved by ventilatory control and by compensating the minor oxygen loss by extra aerial O2 uptake.

Air-breathing behavior in Heterotis niloticus fingerlings: Response to changes in oxygen, temperature, and exercise regimes

Air-breathing in fish is believed to have arisen as an adaptation to aquatic hypoxia. Although air-breathing has been widely studied in numerous fish species, little is known about the obligate air-breathing African bonytongue, Heterotis niloticus. We evaluated if abiotic factors and physical activity affect air-breathing patterns in fingerlings. The air-breathing frequency (fAB ) and behavioral responses of H. niloticus fingerlings were assessed in response to environmental oxygen, temperature, and exhaustion and activity in a series of experiments. The air-breathing behavior of H. niloticus fingerlings under optimum water conditions was characterized by swift excursions lasting less than 1 s to the air-water interface to gulp air. The intervals between air-breaths were highly variable, ranging from 3 to 259 s. Body size only slightly affected fAB , while hypoxia, hyperthermia, and exercise stress significantly increased fAB . Progressive hypoxia from 17.69 to 2.17 kPa caused a ~2.5-fold increase in fAB . Increasing temperatures to 27 and 32°C, from a baseline temperature of 22°C, significantly increased fAB from 0.4 ± 0.2 to 1.3 ± 0.5 and 1.6 ± 0.4 breaths min-1 , respectively. Lastly, following exhaustive exercise, fAB increased up to 3-fold. These observations suggest that H. niloticus fingerlings are very reliant on aerial oxygen, and their air-breathing behavior is sensitive to environmental changes and activity levels.