Dissolved and ~articulate organic matter as possible food sources for eel leptocephali Tsuguo Otakel, Kinya ~o g a r n i~, Keigo ~a r u~a r n a~ ' Ocean Research Institute, University of Tokyo. Tokyo 164. Japan Japan Sea Farming Association, Tamano Station, Tamano, Okayama 706, Japan ABSTRACT: The gut contents and ultrastructure of midgut mucosal cells were examined in the leptocephali of Conger myriaster. Detrital aggregates less than 20 pm in diameter and fecal pellets of zooplankton, which were collectively called 'particulate organic matter' (POM), were found in the gut of 78 "/o of leptocephali examined. These particulate materials were also present in over 90 % of the gut of C. japonicus and Muraenosox cinereus leptocephali which were collected in the same waters. The POM is thought to be a major food item of leptocephali. Gut pigment contents were low in C. rnyriaster, indicating no feeding on phytoplankton. Active seawater ingestion was ultrastructurally indicated in midgut mucosal epithelium, which suggested that dissolved organic matter (DOM) was another possible nutritive source. The stable nitrogen isotopic composition of C. myriaster leptocephali, a reliable indicator of trophic position in food webs, was at the lowest level, equal to that of POM. This result strongly supports our suggestion that POM and DOM are food sources for eel leptocephali.