Lipids in meso- and bathypelagic fishes from the North Atlantic Ocean: dietary inputs suggested from fatty acid trophic markers

Trophic interactions of mesopelagic fishes are key pathways in the vertical transport of carbon through the biological carbon pump. However, diet and feeding behaviours of many members of the mesopelagic community, including lanternfish (Myctophidae) and bristlemouths (Gonostomatidae), remain poorly resolved. We obtained specimens from 1 species of lanternfish, Benthosema glaciale, and several bristlemouths of the genus Cyclothone, from 8 locations in the Northeast Atlantic Ocean, within 19-22° W and 20-55° N, at depths down to 1900 m, and used fatty acid trophic marker (FATM) analysis to assess the main dietary sources and potential feeding habits of those fish. We compared the FA profiles of the fish to those of their zooplankton prey sampled at the same time and locations. The fatty acid composition of the fish separated them into 3 distinct groups. The first group, which included only B. glaciale, was characterized by the phytoplankton trophic markers 16:4 and 18:4(n-3), indicating that the fish had fed on primary consumers, likely zooplankton. The second group included C. microdon and 2 unidentified Cyclothone sp. and was characterized by the monounsaturated FA markers 20:1(n-9) and 22:1(n-11), indicative of lipid-rich calanoid copepods. Finally, the third group, comprising C. pseudopallida and 22 unidentified Cyclothone sp., was characterized by bacterial FA markers (15:0 and 17:0) and saturated FAs (16:0 and 18:0) that, in the deep sea, are likely associated with marine snow. The results show that meso- and bathypelagic fishes sampled at different locations will display differences in FA profiles, reflecting distinct dietary sources. In this way, FATMs may facilitate a better understanding of trophic interactions and energy transfer in deep ocean ecosystems.