Abstract
Siluriform fishes collected from the Rio Negro and Rio Solimões proved to be highly resistant to aquatic hypoxia. In all four species analysed in this study, aquatic oxygen consumption significantly decreased from normoxic levels at water PO 2 values near 1 kPa. Air‐breathing activity was observed only in Sturisoma sp. (Rio Negro). In this species, under severe hypoxia, oxygen uptake from the air dominated, but total oxygen uptake was significantly lower than that under normoxic conditions. In Anadoras weddellii (Rio Solimões), aquatic surface respiration was detected. However, the other species ( Tympanopleura atronasus and three members of the family of Sternopygidae; Rio Solimões) showed no attempt to supplement aquatic oxygen uptake, even under severe hypoxia. In all species tested, neither ammonia nor urea‐N excretion was affected by the decreasing water PO 2 . At the lowest water PO 2 levels, the reduction in total oxygen uptake in the face of unchanged nitrogenous waste excretion resulted in extraordinary high nitrogen quotient (NQ) ratios. In normoxia, NQ ratios ranged from 0.16 to 0.34. Urea‐N excretion contributed between 19% and 28% to total nitrogen excretion and appeared to be unrelated to natural diet as indicated by the gut length‐to‐fork length ratio or to plasma urea‐N levels. Overall, our data underline the quantitative importance of urea‐N for nitrogen excretion in siluriform fishes.