Resumo:
Hypoxic water and episodic air exposure
are potentially life-threatening conditions that fish in
tropical regions can face during the dry season. This
study investigated the air-breathing behavior, oxygen
consumption, and respiratory responses of the
air-breathing (AB) armored catfish Pterygoplichthys
anisitsi. The hematological parameters and oxygenbinding
characteristics of whole blood and stripped
hemoglobin and the intermediate metabolism of
selected tissue in normoxia, different hypoxic conditions,
and after air exposure were also examined. In
normoxia, this species exhibited high activity at night
and AB behavior (2–5 AB h-1). The exposure to acute
severe hypoxia elicited the AB behavior (4 AB h-1)
during the day. Under progressive hypoxia without
access to the water surface, the fish were oxyregulators
with a critical O2 tension, calculated as the inspired
water O2 pressure, as 47 ± 2 mmHg. At water O2
tensions lower than 40 mmHg, the fish exhibited
continuous apnea behavior. The blood exhibited high
capacity for transporting O2, having a cathodic hemoglobin
component with a high Hb–O2 affinity. Under
severe hypoxia, the fish used anaerobic metabolism to
maintain metabolic rate. Air exposure revealed physiological
and biochemical traits similar to those
observed under normoxic conditions.
