Enzyme Activities in the Intestinal Layers of Teleost Fishes: Endogenous versus Exogenous Contribution

Digestive enzymes are essential for the catabolism of macromolecules and are produced endogenously by the animal or exogenously by the gastrointestinal bacteria. This thesis explored the contribution of endogenous versus exogenous trypsin, lipase, and cellulase production across dietary niches by separating the intestines of fed and unfed carnivorous rainbow trout, omnivorous goldfish, and herbivorous central stonerollers into layers where bacteria were present or absent. Trypsin and lipase were both endogenously and exogenously produced in a species-specific manner, whereas cellulase was solely produced by bacteria in all three fishes. As well, feeding generally decreased enzyme activities in most cases. Furthermore, dietary manipulation of goldfish caused the elimination of most trypsin (via endogenous responses) and cellulase (via mostly exogenous responses) activities. Collectively, these findings elucidate the proportional contribution of the microbe host and gastrointestinal bacteria to digestive enzyme production in various fishes with different diets, and provide insight into the digestive functions of environmentally and economically important vertebrates.