Trophic Niche Partitioning Of Sympatric Pelagic Sharks In The Tropical Eastern Pacific Inferred By Fatty Acid Profiles

As the keystone 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 threat 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 effect and the differences of lipid metabolism between sharks and teleosts,in purpose 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 lipoprotein,while those in teleost are carried by albumin and usually have adipose tissues.The oxidation of fatty acids in extra-hepatic 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 off-springs with yolk sac with and without.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.In recent years,the analysis fatty acids(FAs)has emerged as a potentially valuable technology in the field of ecology,including trophic niche ecology,it can be utilized to analysis the inter-and intra-specific in the fields of trophic niche partitioning,allocation patterns and onto-genetic dietary and habits changes,elaborate on:(1)Species coexistence depends on how organisms utilize their environmental resources,namely trophic niche partitioning.The comparative study of inter-specific trophic niches is helpful for understanding the coexistence mechanism of sympatric species.Fatty acid profiles reflect the feeding information of organisms obtaining a relative long time scale,and play an important role of revealing the trophic niche partitioning among species.In this study,we examined liver,plasma,and muscle FAs in five sympatric or coexisting species of oceanic pelagic sharks(blue(P.glauca),silky(C.falciformis),bigeye thresher(A.superciliosus),pelagic thresher(A.pelagicus),and scalloped hammerhead(S.zygaena)),inhabiting the eastern Pacific Ocean.Results showed complex inter-and intra-individual and tissue variation among the 5 shark species.Based on multivariate analysis and mixing models of the muscle FAs,P.glauca and C.falciformis were shown to have the largest FA niche width,indicating diverse feeding habits or habitat isolation,whereas A.pelagicus and A.superciliosus was characterized as occupying a narrower niche width,reflecting increased trophic specialization.Percentages of fatty acid niche overlap indicated strong resource competition between C.falciformis and S.zygaena and a limited degree of dietary isolation between P.glauca and other sharks.Trophic conclusions differed based on the analysis of plasma FAs,which can be attributed to higher dietary FA turnover rates.However,high and unexplained intra-specific variance in liver FAs,made the analysis of trophic niche metrics unreliable.Overall,our multi-targeted approach reveals the magnitude of potential competitive interactions among coexisting tropical shark species and indicates that onto-genetic niche differentiation function as a major factor determining the coexistence of tropical open-ocean sharks.It also expands our understanding of inter-tissue variability and best practices when using fatty acid analysis to estimate trophic niche metrics.(2)There are eight pelagic shark species inhabiting in the tropical Eastern Pacific,including blue shark(P.glauca),silky shark(C.falciformis),oceanic whitetip shark(C.longimanus),bigeye thresher shark(A.uperciliosus),pelagic thresher shark(A.pelagicus),shortfin mako shark(I.oxyrinchus),scalloped hammerhead shark(S.lewini),and smooth hammerhead shark(S.zygaena).To study the differences of their feeding habits,nutritional relationship and trophic niche partitioning,the fatty acid profiles of shark muscle tissues were analyzed.Results showed that I.oxyrinchus occupied the highest trophic level while blue shark occupied the lowest one.The trophic niche overlap of A.uperciliosus,A.pelagicus and I.oxyrinchus indicated their intense prey competition and the spatial segregation compared with other shark species were also detected.A.uperciliosus showed the most niche similarity to A.pelagicus,indicating they may compete for the same resources.P.glauca and C.falciformis occupied the largest trophic niche area indicating their strong environmental plasticity.I.oxyrinchus and S.lewini occupied the smallest trophic niche area,which may imply the specialization of their feeding habits and this was proved by their relatively high trophic levels.(3)C.falciformis occupies the top of the marine food web and regulates the structure of the food web,wheres it has been listed in the IUCN endangered species list due to its overfishing global resources.In this study,for the first time,we used fatty acid profiles to explore the onto-genetic dietary and habits changes of C.falciformis in the tropical East Pacific Ocean.Fatty acid profiles and biomarkers from different individuals are explored as a source of integrated information of their diet.Immature individuals feed in shallow waters close to the coast,while mature ones prefer to feed on mammals and benthic crustaceans along inshore and offshore areas.Due to the demand for reproduction and the supply of buoyancy,female sharks live in offshore upper-middle-class waters.As the opportunistic predator,C.falciformis has feeding preference depends on the availability of prey,lead to the onto-genetic non-significant trophic level transition.In conclusion,this study demonstrated the potential application of fatty acid profiles in shark feeding ecology studies,especially in the fields of inter-specific trophic niche partitioning,coexistence mechanism and intra-specific onto-genetic dietary and habits transition,it is of great significance for revealing sharks’ evolutionary process,understanding their mysterious life history and making effective management strategies for shark conservation.