The Comparative Morphology of the Definitive Swim Bladder in the Catostomidae

The definitive swim bladder of the Catostomidae has been described and its variations noted. It is characteristically the two-chambered type found in cypriniform fishes in general and de- scribed by Tracy as Group 2. The anterior chamber is encased in a tunica externa made up of special connective tissue fibers. This tunic is in two layers and the fibers of these layers are arranged at right angles to each other. It is proposed that this arrangement prevents over-expansion of the anterior chamber and thus maintains free range-of-motion for the Weberian ossicles. The usual catostomid swim bladder is 35 - 45 per cent of the stand- ard length of the individual. Hypentelium, adapted to riffle-type life, has the posterior chamber somewhat reduced. Thoburnia and Pan- tosteus, adapted to mountain streams, have the posterior chamber even more reduced. The Moxostoma genus-group (Moxostoma, Placopharynx, Mega- pharynx and Lagochilia) has a three-chambered swim bladder. Tho- burnia and Hypentelium are retained as separate genera of the Moxo-