Effects of altered ambient pressure on the volume and distribution of gas within the swimbladder of the adult zebrafish, Danio rerio

Many teleosts use gas-filled swimbladders to control buoyancy and influence three-dimensional orientation (pitch and roll). However, swimbladder volume, and its contributions to these functions, varies with depth-related pressure according to Boyle's law. Moreover, the swimbladder volume at a given depth also depends on the compliance of the swimbladder wall, but this latter factor has been investigated in only a limited number of species. In this study, changes in the volume of the zebrafish swimbladder were observed both in vitro and in situ in pressure chambers that allowed simulations of movements within the water column to and from depths of >300?cm. Results show the anterior chamber to be highly compliant, varying ±38% from its initial volume over the range of simulated depths. This large volume change was accomplished, at least in part, by a series of regular corrugations running along the ventral aspect of the chamber wall and another set of pleats radiating from around the communicating duct in the caudal aspect of the chamber wall. The posterior chamber, in contrast, was found to be minimally compliant, varying only a fraction of a percent from its initial volume over the same pressure range. The different volumetric responses of the chambers caused a significant shift in the distribution of gas within the swimbladder system, but only resulted in a change in the whole-body pitch angle of ±2?deg over the range of pressures tested. Together, our findings provide new insights into the control of buoyancy and trim within teleosts and suggest novel mechanisms that may contribute to swimbladder performance.