The Transition to Air Breathing in Fishes: III. Effects of Body Size and Aquatic Hypoxia on the Aerial Gas Exchange of the Swamp Eel Synbranchus Marmoratus

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Abstract
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 PwOO2 (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 (PwOO2 < 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 PwOO2 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 PwOO2 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 PwCOCO2, 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.

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