Environmental Biology of Fishes 36: 93-97, 1993. 0 1993 Kluwer Academic Publishers. Printed in the Netherlands. Cardio-respiratory responses in two ecologically distinct erythrinids (Hoplias mulubaricus and Hoplius lacerdae) exposed to graded environmental hypoxia Francisco Tadeu Rantin’, Mogens L. Glass2, Ana Lucia Kalinin’, Roberto M.M. Verzola’ & Marisa N. Fernandes’ ’ Department of Physiological Sciences, Federal University of Sao Carlos, 13560 Sao Carlos, SP, Brazil ‘Department of Physiology, Faculty of Medicine of Ribeirao Preto, University of Sdo Pa&o, 14049 Ribeirao Preto, SP, Brazil Received 16.7.1991 Accepted 20.1.1992 Key words: Hypoxic bradycardia, Critical oxygen tension, Hypoxic responses, Oxygen extraction, Erythrinids Synopsis Ecologically distinct species of Hoplias were studied as to the cardio-respiratory responses to graded hypoxia. Hoplias malabaricus maintained a constant oxygen uptake down to a PiOZ of 20 mmHg. Oxygen uptake declined markedly at lower PiO, and, concomitantly, cardiac frequency decreased. Concurrent reductions of oxygen uptake and heart rate also occurred in Hoplias lacerdae but at the considerably higher Pi02 of 35 mmHg. These species-specific differences are consistent with the respective habitats: H. malabar- ices occurs in stagnant hypoxic water, whereas H. lacerdae inhabits well-oxygenated rivers. Introduction Erythrinids (order: Cypriniformes) predominantly inhabit shallow oxygen-poor fresh waters in trop- ical and sub-tropical regions of South America (Britiski et al. 1984, Dickson & Graham 1986). Three erythrinid genus can be distinguished al- though their taxonomy is under revision at the species level (Oyakawa 1990). Most erythrinids are considered well adapted to hypoxic conditions as exemplified by the air-breathers Hoplerythrinus unitaeniatus and Erythrinus erythrinus (Randall et al. 1981) and by the exclusive water-breather Ho- plias malabaricus, typical of shallow stagnant lakes (Rantin & Johansen 1984). The genus Hoplias also includes H. lacerdae, occurring in central and southern regions of Brazil. As an exception among erythrinids, this species inhabits well-oxygenated rivers. Consequently, Hoplias malabaricus and Hoplias lacerdae are ecologically distinct species (Godoy 1975), which permits us to relate physio- logical characteristics to habitat differences. In this context, the present study compares cardio-respi- ratory responses to graded environmental hypoxia, including respiratory (fR) and cardiac (fu) frequen- cies. Total oxygen uptake (VOZ), and electrocar- diograms were, moreover, obtained for both spe- cies of Hoplias, exposed to inspired 0, tensions (PiO,‘s) ranging from 140 to 5 mmHg. As a main objective we compared the critical O7 tensions (PcO,‘s) for the reduction of VO, relative to nor- mal levels and for the onset of ‘hypoxic bradycar- dia’ (cf. Holeton 1980).