Lipid metabolism and its physiological and ecological effects of sharks

As the key species, sharks play an important role in regulating the structure and function of the food web of marine ecosystems. In recent decades, due to the continuous increase of global shark catches, the resource status is not optimistic. Eighty percent of marine species are assessed as near endangered by International Union for Conservation of Nature. However, the lack of knowledge on the basic biology, physiology, ecology and other important life history processes of sharks hinders the effective regulation and protection of shark resources. In this paper, we summarized the lipid metabolism pathways of sharks and its effects and the differences of lipid metabolism between sharks and teleosts, in order to further understand the life history of sharks. Shark’s liver is the primary organ where the lipid storage and synthesis, fatty acid oxidation and ketone body formation occur. Digestion and absorption of lipids from diet is the exogenous pathway and lipid synthesis in liver is the endogenous pathway of lipid accumulation of sharks. The lipid metabolism pathways in shark are unique compared with those of the teleost fish. The free fatty acids in shark’s blood are transported by binding with lipoproteins, while those in teleost are carried by albumin and usually have adipose tissues. The oxidation of fatty acids in extrahepatic tissues is limited, which is more dependent on the metabolic capacity of the ketone body which plays a limited role and is only used as alternative sources in the teleost fish. Triacylglycerol and free fatty acid in shark’s liver are the main energy sources, and squalene can provide additional buoyancy. During reproduction, female sharks provide nutrients for their offspring with yolk sac. The fatty acid profiles in the liver and muscles can be used to investigate the trophic ecology on ecological roles, resource partitioning and nutritional physiology of the shark species. Understanding the lipid metabolism pathways of sharks is of great significance for revealing their evolutionary process, understanding their mysterious life history and making effective management strategies for shark conservation.