The permeability properties of the silvery walls of the swimbladders of eels (Conger conger and Anguilla anguilld) have been studied. The intact swimbladder wall is very much more impermeable to carbon dioxide, oxygen and nitrogen than ordinary connective tissue. Removal of the silvery layer of the wall, which contains crystals of guanine with a little hypoxanthine, increases the permeability about 100 times and leaves a transparent ‘epithelial’ layer only a little less permeable than connective tissue to these gases. The wall of the pneumatic duct of the conger swimbladder, which is not silvered, has permeability properties similar to those of connective tissue. In long-lasting experiments a production of carbon dioxide not coupled to oxygen usage was detected in the epithelial layer. The ratios between the diffusion constants of CO, N and O for both the silvery and epithelial layers show that diffusion probably takes place through water-filled pathways, and calculations show that impermeable overlapping crystals embedded in the silvery layer could account for its very low permeability. Two deep-sea eels were shown to have swimbladders whose walls contained about 10 times more guanine/unit area than the conger eel. The possibility that layers of guanine crystals may divide fish into compartments is discussed.
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