MCCUE, M.D.; CHAPMAN, L.J.: African Swamps and Respiratory Tradeoffs: Interdemic Variation in an Air-Breathing Catfish, Clarias Liocephalus

All air-breathing fishes are bimodal and use their air-breathing organs to extract atmospheric oxygen and gills to extract aquatic oxygen, however there is great variation among species in their dependence on aerial respiration. This study was designed to examine whether there is also interdemic variation in respiratory allocation. Given the potentially high costs of air-breathing such as travel and predation risks, it is reasonable to predict higher allocation gill breathing in well-oxygenated waters, and higher allocation to air-breathing in hypoxic habitats. Testing this prediction required quantifying interdemic variation in the size of the air-breathing organ and gill surface area in the air-breathing catfish, Clarias liocephalus: one collected from extremely hypoxic waters of a papyrus swamp in Uganda and the other collected from a well-oxygenated tributary of the swamp. Results reveal that for fish of a given body mass, swamp-dwelling C. liocephalus had air breathing organ masses 42.3 percent greater than fish from the well-oxygenated stream waters, whereas total gill filament length and gill surface area were 12.5 and 12.9 percent smaller, respectively. Smaller gill sizes may reflect the increased space required by the air-breathing organ. Respiratory behaviour also differed between populations. For a fish of given body mass, swamp-dwelling C. liocephalus breathed more frequently across a range of dissolved oxygen levels than a fish from the stream, whereas gill ventilation rates were lower in the swamp-dwelling fish. These results suggest a strong selection pressure for hypoxic tolerance in the swamp habitat may contribute to behavioural and morphological diversification among populations of C. liocephalus.