Air-breathing behavior, oxygen concentrations, and ROS defense in the swimbladders of two erythrinid fish, the facultative air-breathing jeju, and the non-air-breathing traira during normoxia, hypoxia and hyperoxia
The jeju Hoplerythrinus unitaeniatus and the traira Hopliasmalabaricus are two neighboring genera from the family of erythrinid fish, both possessing a two-chambered physostomous swimbladder. In the jeju the anterior section of the posterior bladder is highly vascularized, and the swimbladder is used for aerial respiration; the traira, in turn, is a water-breather that uses the swimbladder as a buoyancy organ and not for aerial oxygen uptake. Measurement of swimbladder oxygen partial pressure (PO2) of fish kept at 26 °C in normoxic, hyperoxic (28–32 mg O2 L− 1) or hypoxic (1–1.5 mg O2 L− 1) water revealed constant values in traira swimbladder. Under normoxic conditions in the jeju swimbladder PO2 was higher than in traira, and the PO2 significantly increased under hyperoxic conditions, even in the absence of air breathing. In jeju, air-breathing activity increased significantly under hypoxic conditions. Hypoxic air-breathing activity was negatively correlated to swimbladder PO2, indicating that the swimbladder was intensely used for gas exchange under these conditions. In traira, the capacity of the ROS defense system, as assessed by measurement of activities of enzymes involved in ROS degradation and total glutathione (GSH + GSSG) concentration, was elevated after 4 h of hyperoxic and/or hypoxic exposure, although swimbladder PO2 was not affected. In jeju, experiencing a higher variability in swimbladder PO2 due to the air-breathing activity, only a reduced responsiveness of the ROS defense system to changing environmental PO2 was detected.