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The Composition and Function of Surface-Active Lipids in the Goldfish Swim Bladder

Goldfish (Carassius auratus: Cyprinidae) have a two-chambered swim bladder. The anterior chamber is thick walled, undergoes small volume changes, does not empty itself of gas, and assists in hearing. The posterior chamber is thin walled, undergoes extensive volume change that often results in bladder collapse, and controls animal buoyancy. Both chambers possess relatively thick inner lipid linings that differ in composition. The anterior-chamber lipids are rich in cholesterol and phospholipids, particularly in disaturated phospholipids. Posterior-chamber lipids are exceptionally rich in cholesterol but not in saturated phospholipid. Lipids from both chambers lower surface tension below that of water but are not more surface active than a cell membrane preparation. This study demonstrates a new technique for determining the role of the lipids in bladder inflation and compliance. The pressure required to initiate the inflation of a collapsed bladder (termed "opening pressure") increased after lipids were removed from the posterior chamber but not after they were removed from the anterior chamber. Posterior-chamber lipids act as an antiglue for collapsed bladders by preventing the folded inner surfaces from adhering. Anterior-chamber lipids did not act as an antiglue, and their function remains unknown.