The Physical Properties of the Swimbladder in Intact Cypriniformes

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Abstract
1. A method is described whereby certain physical properties of the swimbladder have been determined in intact, unoperated Cypriniformes. These properties are volume, excess internal pressure, extensibility, and relative sensitivity (an index of the rate of change of volume with depth). 2. The mean volume of the swimbladder varies in the twelve species examined from 5.8 to 9.9 ml./100 g. body weight. 3. In most Cyprinidae the swimbladder gases are maintained at a mean pressure 2-3 cm. Hg above that of the surrounding water at the depth to which the fish is adapted. Higher pressures occur in a few species, most notably the bream (10.8 cm. Hg). A much lower pressure is found in Amiurus. An excess internal pressure is probably necessary to provide a taut swimbladder wall to operate Weberian ossicles. 4. The swimbladder wall has non-linear elastic properties such that as the excess pressure in the swimbladder rises the extensibility of its wall falls. The swimbladder also shows slow-elastic properties. The significance of these properties is discussed. 5. Excess internal pressure and low extensibility combine to limit the rate at which the volume of the swimbladder, and so the buoyancy of the fish, changes with depth. For small depth changes the swimbladder of the bream (an extreme case) changes its volume only one-quarter as much as would a free air-bubble.

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