Lipids and buoyancy in Southern Ocean pteropods

The lipids of Clione limacina , a Southern Ocean pteropod (order Gymnosomata), contain 28% diacylglyceryl ether (DAGE) (as percentage of total lipid) whereas the pteropod Limacina helicina (order Thecosomata) lacks DAGE. The alkyl glyceryl ether diols (1‐ O ‐alkyl glycerols, GE) of Clione DAGE are dominated by 16∶0 (60%) and 15∶0 (21%), in contrast with deep‐sea shark liver DAGE, which is dominated by 18∶1 GE. The fatty acid profiles of Clione and Limacina are similar (28–32% polyunsaturated, 26–34% monounsaturated) as are the sterols, which include 24‐methylenecholesterol, transdehydrocholesterol, cholesterol, and desmosterol. This finding probably reflects the fact that Limacina is the major food source for Clione. Spongiobranchaea australis , another Southern Ocean pteropod (order Gymnosomata), has 0.9–1.7% DAGE, but has less lipid (3.3–4.8 mg/g lipid, wet weight) than Clione (50.8 mg/g lipid, wet weight). We propose a buoyancy role for DAGE in Clione since Limacina has bubbles for flotation which Clione lack; DAGE provides 23% more uplift than triacylglycerol at a concentration of 1.025 g/mL seawater.