The lipids of six species of shark

Abstract

The compositions of the lipids of egg yolk, blood plasma and liver ofScyliorhinus caniculafrom the Irish Sea were examined by column chromatography and by various other methods. Considerable differences in composition were found between these three tissues. Phospholipid and triacylglycerols were the major components of lipids of yolk and liver respectively. Cholesterol, free fatty acid and phospholipid were present in all three tissues. Squalene and alkyldiacylglycerols were present only in traces, if at all. In this paucity or absence of squalene, this population differs from MediterraneanS. canicula. The compositions of the lipids of the livers of six shark species (Scyliorhinus canicula, Scyliorhinus stellaris, Squatina squatina, Galeorhinus galeus, Mustelus asteriasandSqualus acanthias) were compared by thin-layer chromatography. In all cases, triacylglycerols were the major components, while hydrocarbons were not detected by this method in any of the species. Alkyldiacylglycerols were present as a major component only inSqualus. Qualitatively, the other five species showed only minor differences from each other; these five species are largely confined to shallow coastal waters. The results follow previous work in showing that low-density lipids, such as squalene and alkyldiacylglycerols, are major components of hepatic lipid in shark species from deeper waters, although not necessarily from great depths. An attempt is made to explain the prevalence of such lipids in pelagic sharks in evolutionary terms. It is suggested that they may serve as an analogue of the swimbladder of teleosts.