Buoyancy in marine fishes: direct and indirect role of lipids

Abstract

The major lipids that have a direct role in buoyancy of marine fish are wax esters, squalene, and alkyldiacylglycerols. Wax esters are stored extracellularly in certain fishes, such as the orange roughy (Hoplostethus atlanticus), and therefore buoyancy appears to be their sole function. Some myctophid fishes have wax-invested swimbladders, where the non-compressible wax esters may aid in diurnal vertical migration, by replacing compressible swimbladder gases. Squalene is metabolically inert in the livers of certain sharks, and therefore probably has buoyancy as its only function. Alkyldiacylglycerols (DAGE) are abundant components of liver oil of certain deep sea sharks and holocephalans, where they may have an important role in buoyancy. Triacylglycerols and cholesterol are lipids that have an indirect role in buoyancy of marine fish. Many fishes in the ocean have oil-filled bones (mostly triacylglycerols). Although this oil aids buoyancy, its major function is as an energy storage lipid which can be utilized during starvation. Cholesterol, which is found in high amounts in the lipid-rich membranes of the swimbladder of deep sea fishes, may aid buoyancy by combining with oxygen gas in the swimbladder membranes to facilitate gas secretion in fish at great depths in the ocean. Further research is needed to understand the physical state of lipids, such as wax esters at deep sea temperatures and pressures, and more evidence is needed to clarify the role of cholesterol-rich membranes in swimbladders of deep sea fishes.