Analyses of gut content and isotopic composition of Anguilliformes leptocephali near southern Japan

Unlike most fish larvae that prey exclusively on mesozooplankton, such as copepods, leptocephali feed on a type of particulate organic matter (POM) known as marine snow. However, the components of POM consumed and assimilated by leptocephali have not been clarified to date. To characterize POM components ingested in the form of marine snow by leptocephali found in the waters around Satsunan, southern Japan, we performed morphological and DNA metabarcoding analyses of their gut contents and stable carbon and nitrogen isotope analyses for leptocephali and their potential food items. The analyses were carried out on the Dysomma, Gnathophis, and Muraenosox leptocephali, which occur abundantly in this area. Our data supported that leptocephali feed on marine snow particles, which can contain faecal pellets of copepods. The gut contents varied greatly among taxonomic groups of leptocephali. Differences in feeding depth and the transport of larvae from other regions may lead to differences in the leptocephali gut contents. Our results showed that the leptocephali of the Satsunan area did not actively use gelatinous zooplankton (i.e. cnidarians) and appendicularian houses, whose presence was frequently confirmed in leptocephali guts by former studies, as nutrient sources or as substrates to aggregate small particles such as bacteria, protozoans, and zooplankton faecal pellets. These results suggest that leptocephali feed non-selectively on marine snow in the ambient water column.