Improved ROS defense in the swimbladder of a facultative air-breathing erythrinid fish, jeju, compared to a non-air-breathing close relative, traira

Abstract

The jeju Hoplerythrinus unitaeniatus and the traira Hoplias malabaricus are two closely related 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. Observation of the breathing behavior under different levels of water oxygenation revealed that the traira started aquatic surface respiration only under severe hypoxic conditions and did not breathe air. In the jeju air-breathing behavior was observed under normoxic conditions, and the frequency of air-breathing was significantly increased under hypoxic conditions. Unexpectedly, even under hyperoxic conditions (30 mg O2 L(-1)) the jeju continued to take air breaths, and compared with normoxic conditions the frequency was not reduced. Because the frequently air-exposed swimbladder tissue faces higher oxygen partial pressures than normally experienced by other fish tissues, it was hypothesized that in the facultative air-breathing jeju, swimbladder tissue would have a higher antioxidative capacity than the swimbladder tissue of the water breathing traira. Measurement of total glutathione (GSSG/GSH) concentration in anterior and posterior swimbladder tissue revealed a higher concentration of this antioxidant in swimbladder tissue as compared to muscle tissue in the jeju. Furthermore, the GSSG/GSH concentration in jeju tissues was significantly higher than in traira tissues. Similarly, activities of enzymes involved in the breakdown of reactive oxygen species were significantly higher in the jeju swimbladder as compared to the traira swimbladder. The results show that the jeju, using the swimbladder as an additional breathing organ, has an enhanced antioxidative capacity in the swimbladder as compared to the traira, using the swimbladder only as a buoyancy organ.

Keywords: Erythrinid fish; Oxygen partial pressure; ROS defense; Reactive oxygen species; Swimbladder.

Fig. 1

A Air-breathing activity in the jeju in an individual experiment with progressive hypoxia. Each dashed blue line indicates a single air-breath. Air-breathing frequency was significantly elevated under hypoxic conditions. B Aquatic surface respiration activity (ASR) in an individual traira. In traira air-breathing was never observed. Under severe hypoxia traira started ASR. Each dashed red line indicates a single ASR event. Black dots indicate individual oxygen measurements documenting the declining PO₂.Observations ended after the final PO₂ measurement (color figure online)

Fig. 2

Quantitative analysis of the air-breathing and ASR activity of jeju (N = 10) and traira (N = 9). A Number of air-breaths per hour in jeju under normoxic, hypoxic and hyperoxic conditions. B Number of ASR events per hour in traira under normoxia, hypoxia and hyperoxia. C, D % of time spent at the surface for air-breathing or ASR in jeju (C) and traira (D), respectively. Small letters denote significant differences between normoxia, hypoxia and hyperoxia (p < 0.05). Bars without letters are not different from each other

Fig. 3

Total GSSG/GSH concentration in anterior and posterior swimbladder tissue and in muscle tissue of jeju and of traira. Small letters denote significant differences between tissues within a species, capital letters denote significant differences between the two species (N = 6; p < 0.05)

Fig. 4

Catalase activity in U mg⁻¹ protein in anterior and posterior swimbladder tissue and in muscle tissue of jeju and of traira. Capital letters denote significant differences between the two species, bars without letters are not significantly different; asterisk indicates significant overall difference between the two species (jeju, N = 11; traira, N = 9; p < 0.05)

Fig. 5

SOD activity in U mg⁻¹ protein in anterior and posterior swimbladder tissue and in muscle tissue of jeju and of traira. Capital letters denote significant differences between the two species, bars without letters are not significantly different (jeju, N = 11; traira, N = 10; p < 0.05)

Fig. 6

Glutathione reductase (A) and glutathione peroxidase (B) activity in mU mg⁻¹ protein in anterior and posterior swimbladder tissue and in muscle tissue of jeju and of traira. Small letters denote significant differences between tissues within a species, capital letters denote significant differences between the two species, bars without letters are not significantly different, asterisk indicates significant overall difference between the two species (N = 8; p < 0.05)