Coordination of Respiratory and Hydrostatic Functions of the Swimbladder in the Central Mudminnow, Umbra Limi

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Abstract
Observations of behaviour and changes in buoyancy of Umbra limi, a facultative air-breathing fish, were studied to understand coordination of respiratory and hydrostatic functions of the swimbladder. Fish were exposed to either normoxic or hypoxic water in either undisturbed or disturbed (simulating predator presence) conditions. Declines in swimbladder volume occurred between air-breaths as O2 was removed. These varied between treatments averaging 1.3% in disturbed normoxic conditions, 4.1% and 6.4% in undisturbed treatments (normoxic and hypoxic conditions respectively), and 8.3% in disturbed hypoxic conditions. To minimize the extent and rate of such changes and thereby reduce energy costs of a non-optimal buoyancy, fish either maintained a continuous positive buoyancy at the water surface, compressed the swimbladder gases after inspiration and gradually reduced the pressure to compensate for O2 uptake, or increased their reliance on aquatic O2. The use of any of the above mechanisms was determined by the amount of dissolved O2 and presence or absence of disturbance. In normoxic water fish without access to the surface maintained neutral buoyancy despite a very limited ability to secrete swimbladder gases. The frequency of air-breathing in normoxic water was independent of swimbladder O2 levels, indicating that fish breathe air in normoxic water in response to a decline in swimbladder volume. The potentially conflicting roles of the swimbladder are well co-ordinated.

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