Digestive response and rates of growth in pre-leptocephalus larvae of the Japanese eel Anguilla japonica reared on artificial diets

Abstract

As food protein digestion is instrumental for promoting growth, the main protease in young marine fish larvae, trypsin, was studied in pre-leptocephalus larvae of Anguilla japonica. Tryptic enzyme activity was monitored until day 24 and rates of growth until day 36 after hatch in larval A. japonica derived from artificially matured parent fish. Tryptic activity increased with larval age and developmental stage until day 16, after which tryptic levels stabilized. In start-feeding larvae, tryptic activities increased after ingestion of an artificial diet, but elevated tryptic activities could also be found in larvae with guts void of prey, possibly due to intestinal retention of trypsin secreted in response to ingestion of a previous meal. Gut retention time for trypsin was estimated to be at least 15 h. The gut evacuation time for the artificial diet depended on the meal size and was in the range of 1–5.5 h. Rates of larval growth in length were ca. 5% day −1 during the yolk-sac stage, declining to ca. 1% day −1 in older larvae. Vision was highly important for initiation of feeding. Results provide the first quantitative information on aspects of protein digestion in eel larvae.