Respiratory responses of the air-breathing fish Hoplosternum littorale to hypoxia and hydrogen sulfide

The present study analyzes the respiratory responses of the neotropical air-breathing fish Hoplosternum littorale to graded hypoxia and increased sulfide concentrations. The oxygen uptake (V˙O2), critical O2 tension (PcO2), respiratory (fR) and air-breathing (fRA) frequencies in response to graded hypoxia were determined for fish acclimated to 28 °C. H. littorale was able to maintain a constant V˙O2 down to a PcO2 of 50 mm Hg, below which fish became dependent on the environmental O2 even with significant increases in fR. The fRA was kept constant around 1 breath h− 1 above 50 mm Hg and increased significantly below 40 mm Hg, reaching maximum values (about 4.5 breaths h− 1) at 10 mm Hg. The lethality to sulfide concentrations under normoxic and hypoxic conditions were also determined along with the fRA. For the normoxic fish the sulfide lethal limit was about 70 μM, while in the hypoxic ones this limit increased to 87 μM. The high sulfide tolerance of H. littorale may be attributed to the air-breathing capability, which is stimulated by this compound.