Geographic variation in stable isotopic and fatty acid composition of anguilliform leptocephali and particulate organic matter in the South Pacific

The feeding ecology of leptocephali has remained poorly understood because they apparently feed on particulate organic matter (POM), which varies in composition, and it is unclear which components of the POM they assimilate. The delta C-13 and delta N-15 stable isotope (SI) and fatty acid (FA) compositions of 3 families of leptocephali and POM were compared in 3 latitudinal current zones of the western South Pacific. The delta N-15 signatures of leptocephali and POM overlapped, with both having their lowest values in the southern current zone. POM in general (across all zones) contained 38 FAs and was rich in saturated FAs (SFA) (16: 0, 18: 0, 14: 0), while leptocephali contained 50 FAs, with high proportions of 16: 0, and higher contributions of 22:6 omega 3, 20: 5 omega 3, 18:1 omega 9, 16:1 omega 7 and other FAs than found in the POM. Serrivomeridae leptocephali in the north had higher delta N-15 signatures and were also distinguished from Nemichthyidae and Murae nidae larvae by their FA compositions (higher SFAs, lower 22: 6 omega 3 and 20: 5 omega 3). Although SI signatures of the Serrivomeridae larvae did not clearly vary with size, 16:0 and 18:0 FA proportions decreased with increasing larval size, and 22: 6 omega 3 and 16: 1 omega 7 increased in larger larvae. Correspondences between the latitudinal variations in nitrogen SI signatures and FA compositions of POM with those of leptocephali and the presence of FA markers of both autotrophic and heterotrophic organisms were consistent with leptocephali feeding on POM. POM can contain various materials from primary producers and heterotrophic microorganisms, but differences in the SI signatures and FA compositions in leptocephali remain to be explained through further research.