Fatty acid biomarkers reveal niche separation in an Arctic benthic food web

Marine fatty acids (FA) are synthesized primarily by phytoplankton and have a wide variety of structures. Biochemical restrictions on the synthesis or modification of FA in animals make it possible to recognize those derived from their prey; numerous studies have demonstrated the transfer of FA from prey to predator at various trophic levels. Although rarely occurring, a few FA found in animals, including the C-20 and C-22 non -methylene -interrupted (NMI) FA, can be traced to quite specific prey types. These unusual lipids are synthesized de novo by certain benthic mollusks, particularly bivalves, through chain elongation and desaturation of common monounsaturated FA precursors. Their proportions vary among species, suggesting that they might be particularly suitable as biomarkers in food web studies. We recently discovered NMI FA in 2 species of sympatric, benthicfeeding pinnipeds in Alaska, bearded seals Erignathus barbatus and Pacific walruses Odobenus rosmarus. Significant differences in proportions of 6 NMI FA, and patterns of other FA, demonstrated considerable dietary separation between the bearded seals and walruses. This is the first report of NMI FA in marine mammals, and the first use of these specific FA biomarkers in evaluating trophic pathways in mammals. These individual FA biomarkers, together with suites of other FA, have wide application in understanding marine food webs, including those in the Arctic where rapid environmental change threatens fragile ecosystems.