GILL MORPHOMETRY OF THE FACULTATIVE AIR-BREATHING LORICARIID FISH, HYPOSTOMUS-PLECOSTOMUS (WALBAUM) WITH SPECIAL EMPHASIS ON AQUATIC RESPIRATION

Gill respiratory surface area and oxygen consumption during aquatic re spiration were measured in the facultative air-breathing loricariid fi sh, Hypostomus plecostomus. The fish did not surface to breathe atmosp heric air in normoxic water; air-breathing was evoked by environmental hypoxia (water oxygen tension = 35+/-2 mmHg) and did not show size-re lated threshold differences for air breathing. During gradual hypoxia, without access to atmospheric air, H. plecostomus was found to be an oxyregulator and showed a reduced range of water oxygen tension in whi ch the oxygen consumption remained constant in smaller fish. The criti cal oxygen tensions were 55 and 33 mmHg at 25 degrees C for fish of 14 -30g and 31-80g body weight, respectively. The gill respiratory surfac e area (total lamellae area) is reduced, however, the lamellar frequen cy per mm of gill filament is high which facilitates the gas exchange. Moreover, the increase of gill respiratory surface area (b = 0.666) i s higher than the increase in routine VO2 (b = 0.338) showing a positi ve relationship between the gill respiratory surface area /VO2 ratio a nd body mass (b = 0.328); this indicates that the fish have greater gi ll respiratory surface area per unit of routine VO2 as they grow.