ABSTRACT
The swim (gas) bladders come in two kinds: physostomous and physoclistous. The swim bladder consists of three layers: tunica externa, submucosa, and mucosa. Numerous blood vessels penetrate the tunica externa that constitute a widely distributed rete mirabile. The gas gland is separated from the lumen of the swim bladder by an inner epithelial layer and smooth muscles. This close association of smooth muscles with the gas gland tissue may be important for gas secretion. The gas gland originates from the gas bladder (GB) epithelium, which is formed of folded columnar epithelial cells highly vascularized by long loops of densely packed capillaries, a rete mirabile. The rete in companion with the gas gland permits the gas secretion into the GB. Furthermore, the gas secretion in the bladder is possible by the countercurrent organization of the venous and arterial capillaries of the rete mirabile. Despite the fact that the hydrostasis is the main function of the gas-filled organs in most bony fishes, these organs may have a respiratory role.
