Respiratory patterns and antipredator responses in the central mudminnow, Umbra limi, a continuous, facultative, air-breathing fish

The central mudminnow. Umbra limi, is a continuous facultative air breather whose respiration is primarily aquatic in normoxic water and primarily aerial in hypoxic water. Under these conditions the frequency of respiration (air breaths; branchial breaths) by the primary mode increases with temperature. In hypoxic water, fish exposed to simulated predator disturbance breathed air in synchrony where a breath by one fish was immediately followed by breaths from one or more other fish. Undisturbed fish breathed air at random times with respect to other individuals. The level of dissolved O2 at which fish switch from primarily aquatic to primarily aerial respiration during progressive hypoxia was positively related to temperature. When fish were exposed to progressive hypoxia in groups (n = 10) the transition to air breathing in terms of dissolved O2 concentration was unaffected by acclimation to hypoxia and by simulated predator disturbance. When held alone (isolated) and disturbed, fish became very active and switched to aerial respiration at a higher level of dissolved O2 than either fish held alone and undisturbed or fish held in a group of 10. During progressive hypoxia without access to air, mudminnows maintained both a high level or activity and frequency of branchial breathing down to 15 Torr (1 Torr = 133.322 Pa). Acclimation to hypoxia did not greatly increase resistance to hypoxia in fish without access to air.