Development of the digestive system in red drum (Sciaenops ocellatus) larvae

The development of microparticulate diets (MPD) to replace live foods in the culture of marine fish larvae requires a thorough understanding of digestion processes occurring during ontogeny. This study examined fundamental and applied aspects of the development of the digestive system in red drum larvae. Morphology and histology of the digestive system are described and related to their functional characteristics. Two major periods of differentiation could be distinguished. During the first period, within three days of hatching and prior to exogenous feeding, the digestive tract exhibited extensive differentiation from a straight tube to a looped alimentary canal with clearly distinguishable foregut, midgut and hindgut regions. The second period corresponded to the formation of the pyloric cecae and stomach around 22 days post-hatch. Digestive capacity of the larvae was evaluated by quantifying and characterizing digestive enzyme activities using biochemical and substrate-SDS-PAGE techniques with specific inhibitors. Enzymes for the digestion of proteins, lipids and carbohydrates were present before exogenous feeding commenced, and subsequently increased with age and length. Red drum possessed a nearly complete set of alkaline proteases since first feeding that played a major role in digestion during early development. Acid proteases became more important toward the end of the larval period, concomitant with the appearance of a functional stomach. The effect of feeding regime (live food and/or MPD) on digestive enzyme activity was evaluated. MPD alone resulted in lower growth and survival, but no diet-induced differences in activity of pancreatic enzymes were detected. Lastly, the effect of adding algae to the culture tanks of larvae led live food or MPD from first feeding was examined. The presence of algae increased growth, survival and enzyme activity for both types of feeds. Growth of larvae fed the MPD in the presence of algae was not significantly different from the live food treatments. Results demonstrated that red drum larvae may be raised on a MPD from first feeding without the use of zooplankton. Based on digestive enzyme characterization and observed differentiation of the digestive tract and associated organs, it appears that the digestive system in red drum is highly functional at first feeding.