The hydrogen sulfide (H2S) is a highly toxic compound
binding cytochrome c oxidase, being considered a potent
inhibitor of the aerobic respiration (Bagarinao and Vetter,
1989). It results mostly from bacterial sulfate reduction in
sediments and anaerobic decomposition of the organic
matter (Jorgensen, 1984) and is commonly found in
floodplain lakes (‘‘varzeas’’) in the Amazon region
(Affonso, 2000) and Pantanal Matogrossense (mid east
Brazil) where H. littorale is an important component of the
fish communities. Tamoata´ is one of the few species able to survive the combined effects of environmental hypoxia and
high sulfide concentrations when the thermal stratification
of small floodplain lakes is disrupted (Affonso, 2000).