Pathological Effects of Soybean Anti-Nutritional Factors on Summer Flounder (Paralichthys Dentatus) Tissues
Summer flounder (Paralichthys dentatus), a popular carnivorous fish in New England, is an important candidate for aquaculture development. The inclusion of plant proteins as a replacement for fish meal in the diets of marine carnivorous fish may lead to economical advantages and increased sustainability. Anti-nutritional factors, organic molecules that cannot be digested and may inhibit digestion of other molecules present in soybean meal, but not in soy protein concentrate, may limit the inclusion of soybean meal into carnivorous fish diets by impacting fish growth rates or immune function. In order to determine the mechanisms by which soybean meal impacts growth or immune function, it is important to analyze the effect of anti-nutritional factors on the morphology of important digestive and immune organs: liver, spleen, and intestine. The goal of this project was to determine: 1) If pathological change was occurring in selected summer flounder organs when fish were fed diets in which a portion of fish meal was replaced with soy protein concentrate and varying amounts of anti-nutritional factors; and 2) Which fractions of soybeans (either as saponin-containing or oligosaccharide-rich), led to pathological changes. Feeding of summer flounder for eight weeks with diets in which 60% of fish meal was replaced with soy protein concentrate supplemented with increasing amounts of a fraction of soybean flakes containing anti-nutritional factors (corresponding to the amounts present in a 5%, 14%, and 27% soybean meal replacement diet) led to a significant decrease in growth in all diets compared to that with a fish meal control diet. Fish fed diets containing anti-nutritional factors at levels as low as those present in a 5% soybean meal replacement diet showed significant pathological changes in liver, spleen, and anterior intestinal morphology as early as two weeks into the trial. These changes included: a decrease in the storage of nutrients in liver, a relative increase in the amount of white pulp versus red pulp and the presence of fibrosis in the spleen, and a decrease in the amount of goblet cells in the anterior intestine, accompanied by an increase in the thickness of the lamina propria and fusion and shortening of the mucosal folds. Fish fed the 27% diet had the worst overall growth and the most apparent change in tissue morphology, suggesting that anti-nutritional factors in soybean meal have a dose-dependent impact on the liver, spleen, and anterior intestine of summer flounder. A second six-week feeding trial was conducted in order to determine the impact of soy saponins and oligosaccharides on fish growth and tissue morphology. There were no statistically significant changes in morphology in all parameters evaluated except in the thickness of the lamina propria in the anterior intestine. Therefore, low levels of soy saponins and oligosaccharides may not significantly impact the morphology of summer flounder spleen, liver and anterior intestinal tissue. Pathological changes observed in fish fed the soybean meal equivalent replacement diets may be due to higher amounts of anti-nutritional factors in these diets or to additive or synergistic impacts of several anti-nutritional factors.