Observations of gut contents of anguilliform leptocephali collected in the western North Pacific

The gut contents of anguilliform leptocephali collected by research cruises in the western North Pacific were examined in order to investigate their food sources. Gut contents of 26 leptocephali (Anguillidae, Congridae and Serrivomeridae) examined were amorphous food material containing many small spherical particles. Alcian Blue (AB)-stained aggregates containing spherical particles approximately 2-10 mu m in diameter were observed in 14 leptocephali from the 2015 survey, and Coomassie Brilliant Blue (CBB)-stained aggregates containing spherical particles about 2 mu m in diameter were observed in 12 leptocephali from the 2016 survey. Red autofluorescence of phytoplankton was exhibited in several sized cells in amorphous food material. Furthermore, polysaccharide-containing transparent exopolymer particles (TEP), proteinaceous Coomassie stainable particles (CSP), and pico/nano-planktonic particles about 2 mu m in diameter were observed in environmental water samples. These results suggest that anguilliform leptocephali utilize phytoplankton-derived particulate organic matter (POM) as one of the food sources. Such feeding ecology makes it possible for anguilliform leptocephali to avoid food competition from predatory, zooplanktivorous and omnivorous fishes that have higher trophic positions in the oligotrophic ocean.