Feeding ecology of anguilliform leptocephali considering the structure and proximate composition of food organisms and gut contents

Scanning electron microscope (SEM) observations and component analyses were conducted on the gut contents of wild-caught larvae and cultured larvae that fed on available microorganisms in the water to determine the physical characteristics and nutritional contributions of biological species that are presumed to be food elements of anguilliform leptocephali. There were no traces of food organisms, such as autofluorescence of algae and faecal pellets, or identifiable objects from zooplankton in the gut contents of wild larvae. The gut contents of both wild and cultured larvae were light brown sols or gels and were composed of amorphous substances consisting mainly of carbohydrates, proteins and amino acids. The structural properties of the gut contents of cultured larvae were similar to those of particulate organic matter (POM) in rearing water, and mucus secreted from the intestinal epithelium was also present. These results suggest that low-molecular-weight saccharides, proteins and amino acids produced by algae and bacteria may be among the available nutritional sources of eel larvae and that the mucus layer of the intestinal epithelium contributes to a feeding mode that effectively captures microparticles from environmental water. Taken together, the results of recent surveys suggest that pico- and nanosized POMs, which are ubiquitous in environmental waters and are easily ingested, digested and are absorbable, are a food source of leptocephali. These substances produced by algae and bacteria that account for a high proportion of marine biomass and dissolved organic matter (DOM), may be directly utilized by the intestinal epithelium through pinocytosis without microbial degradation.