Lipid content and composition of three species of Antarctic fish in relation to buoyancy
The lipip content and composition of various tissues from three species of nototheniid fish from McMurdo Sound, Antarctic have been examined in relation to their habitat and buoyancy. The pelagic midwater Dissostichus mawsoni is neutrally buoyant. It is rich in lipid which is located subcutaneously, as adipose tissue associated intimately with white muscle, and as lipid droplets within the cells of various tissues. White muscle, red muscle and liver are particularly lipid-rich, although the liver is not positively buoyant. The amount of lipid stored in the white muscle increases towards the centre of buoyancy of the fish. These deposits are documented at the anatomical, histological and ultrastructural levels. Tissues of Pagothenia borchgrevinki contain less lipid than D. mawsoni, but liver, red muscle and white muscle are still very rich in lipid. This species is cryopelagic, that is it spends most of the time in the water column just beneath the surface ice layer. It is not neutrally buoyant, but has a low weight in seawater. The tissues of the benthic Trematomus bernacchii contain only normal levels of lipid. The lipid class compositions of all three species are dominated by triacylglycerol, particularly when lipid contents are high. Serum lipids are an exception in containing high levels of the transport lipid sterol ester. The reason why Antarctic fish use triacylglycerols for buoyancy rather than was esters (as used by many myctophids) or squalene (as used by some sharks) is unclear.