Static Stability and Swim Bladder Volume in the Bluegill Sunfish (Lepomis macrochirus)

Static stability is a property inherent to every organism. More stable bodies benefit from a lower energy cost associated with maintaining a desired orientation, while less stable bodies can be more maneuverable. The static stability of a fish is determined by the relative locations of its center of mass (COM) and center of buoyancy (COB), which may change with changes in swim bladder volume. We hypothesized, however, that fish would benefit from consistent static stability, and predicted that changes in swim bladder volume would not alter the overall pattern of COM and COB locations. We used micro-computed tomography to estimate the locations of the COM and COB in bluegill sunfish (Lepomis macrochirus). Using this technique, we were able to find a small but significant difference between the location of the COM and COB for a given orientation. We found that the swim bladder can change shape within the body cavity, changing relative locations of the COM and COB. At one extreme, the COB is located 0.441 ± 0.007 BL from the snout and 0.190 ± 0.010 BL from the ventral surface of the pelvic girdle, and that the COM is 0.0030 ± 0.0020 BL posterior and 0.0006 ± 0.0005 BL ventral to the COB, a pattern that causes a nose-up pitching torque. At the other extreme, the COM is anterior and dorsal to the COB, a pattern that causes the opposite torque. These changes in location seems to be caused by changes in the shape and centroid location of the swim bladder within the body: The centroid of the swim bladder is located significantly more posteriorly in fish oriented head-down. The air in the bladder "rises" while heavier tissues "sink," driving a change in tissue distribution and changing the location of the COM relative to the COB. Supporting our hypothesis, we found no correlation between swim bladder volume and the distance between the COM and COB. We conclude that bluegill are statically unstable, requiring them to expend energy constantly to maintain their normal orientation, but that the pitch angle of the body could alter the relative locations of COM and COB, changing their static stability.