Printed in Great Britain SWIMBLADDER PERMEABILITY TO OXYGEN

The permeability of the swimbladder to gas was measured as the conductance to oxygen in seven species of fish. The low over-all conductance resides in the low diffusion constant, K, of the middle layer of the wall (submucosa) while the outer and inner layers (tunica externa and mucosa) have diffusion constants similar to other vertebrate tissues. The low diffusion constant is due to a low diffusion coefficient, D (not to a low-solubility coefficient), apparently caused by multiple layers of very thin (about 0-02 /im) and broad (up to 100 /im) crystals, clearly identifiable in electron micrographs, and presumably consisting of guanine. Crystals occupy only a small fraction of tissue volume. They are flexible as a consequence of their extreme thinness, and subject to being elaborately folded when the mucosa and submucosa of the secretory part of the swim-bladder is contracted during reabsorption of gas. The low conductance of the swimbladder wall reduces the metabolic energy needed to maintain swimbladder volume