Summary

Synbranchus marmoratus (Bloch) breathes air during terrestrial excursions and while dwelling in hypoxic water and utilizes its gills and adjacent buccopharyngeal epithelium as an air-breathing organ (ABO). This fish uses gills and skin for aquatic respiration in normoxic (air-saturated) water but when exposed to progressive aquatic hypoxia it becomes a metabolic O₂ conformer until facultative air breathing is initiated. The threshold Pw_OO2 (aquatic O₂ tension or partial pressure in mmHg) that elicits air breathing in S. marmoratus is higher in larger fish. However, neither air-breathing threshold nor the blood haemoglobin (Hb) concentration of this species were changed following hypoxia (Pw_OO2 < 20 mmHg) acclimation. In hypoxic water S. marmoratus supplies all of its metabolic O₂ requirement through air breathing. ABO volume scales with body weight raised to the power of 0.737 and the amount of O₂ that is removed from each air breath depends upon the length of time it is held in the ABO. Ambient Pw_OO2 directly affects the air-breath duration of this fish, but the effect is smaller than in other species. Also, average air-breath duration (15.7 min at Pw_OO2 0–20 mmHg) and the average inter-air-breath interval (15.1 min) of S. marmoratus are both longer than those of other air-breathing fishes. Although the gills of S. marmoratus are involved in aerial O₂ uptake, expelled air-breath CO₂ levels are not high and always closely correspond to ambient Pw_COCO2, indicating that virtually no respiratory CO₂ is released to air by this fish. CO₂ extrusion therefore must occur aquatically either continuously across another exchange surface or intermittently across the gills during intervals between air breaths. This study with S. marmoratus from Panama reveals physiological differences between this population and populations in South America. The greater Hb content of South American S. marmoratus may be the result of different environmental selection pressures.