Diets supplemented with limonene and thymol modify intestinal histomorphology of Nile tilapia Oreochromis niloticus

Abstract

Diets can influence the histomorphology of the gut of fish by either causing changes that are associated with growth promotion or causing damage that impairs fish growth. This study evaluated effects of diets with limonene and thymol on the histomorphology of the anterior intestine of Nile tilapia. A Nile tilapia diet was supplemented with different concentrations of limonene and/or thymol to form 6 diets, namely: 0 ppm limonene and thymol (Control); 400 ppm limonene (LM); 500 ppm thymol (T1); 750 ppm thymol (T2); 400 ppm limonene and 500 ppm thymol (LMT1); and lastly 400 limonene and 750 thymol (LMT2). Each diet was assessed in four tanks, each stocked with 35 fish of 1.60 g ± 0.24 and fed to satiation for 56 days. The length and number of intestinal folds, diameter of the lumen, status of lamina propria, and size of sub-mucosa were evaluated. Fish fed on diet LM and LMT1 had longer intestinal folds suggesting that diet LM and LMT1 increased the surface area covered by the intestinal folds and this is associated with increased nutrient absorption. The number of intestinal folds was not affected by the test diets. Treatment LMT1 had significantly narrow intestinal lumen potentially due to the elongated intestinal folds observed with dietary LMT1. Fish fed on diet T2 had enlarged lamina propria infiltrated with cells while the sub mucosa was larger in the anterior intestine of fish fed on diet LMT2 and the control feed. Enlarged lamina propria and intestinal sub mucosa indicate an inflammatory response and inefficient passage for blood and nutrients in the enterocytes in the intestine, potentially affecting efficient nutrient absorption. Overall, the diet with 400 ppm limonene and the dietary combination of 500 ppm thymol and 400 ppm limonene increased the surface area covered by intestinal folds indicating that they can enhance nutrient absorption in juvenile Nile tilapia.