Adaptive Variation in Swimbladder Length and Volume in Dace, Genus Rhinichthys

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Abstract
Relative growth in length and volume of swimbladder is described in four species of dace, genus Rhinichthys. Swimbladder volume varied widely in R. atratulus, R. cataractae, and R. osculus as they adjusted buoyancy by altering swimbladder volume, but variability was less in R. falcatus whose range was overlapped by the other three species. A variety of relative growth patterns in swimbladder length was found due mainly to the presence or absence of an inflection which, when present, varied at fork length of occurrence. Where ecological information was known, the length at occurrence of inflection corresponded to a change in environment occupied, from still to current. In the laboratory, R. atratulus and R. cataractae adjusted swimbladder volume to compensate for different water velocities. All age-groups of the former attained a greater buoyancy than the latter, while R. cataractae reached a lower negative buoyancy. Young R. cataractae made greater adjustments of swimbladder volume than older fish; the reverse occurred with R. atratulus. These adjustments resulted in a significant change in swimbladder length independent of age, except for R. cataractae from Lake Winnipeg which had very short swimbladders. The hypothesis that occurrence of the inflection in swimbladder length was caused by the shift from a still to current environment, resulting in a decrease in swimbladder volume and length was accepted for R. atratulus, but rejected for R. cataractae. An additional hypothesis based on environmental control is proposed.

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