Effect of increasing levels of Hermetia illucens in a fishmeal-free diet at sea bream (Sparus aurata, L.) gastrointestinal level

The impact of a fishmeal-free, high plant-protein-based diet, in which vegetable proteins (VPs) were partially substituted with the proteins derived from Hermetia illucens pupae meal (Hi) was examined in the gilthead sea bream (Sparus aurata). The physiological response at the gastrointestinal level was investigated by considering the gene expression responses and histology at the liver, stomach, pyloric caeca, foregut, midgut, and hindgut. To this end, three iso-proteic (crude protein, Nx6.25, 45 %) and iso-lipidic (crude fat, 20 %) extruded diets were formulated. A control diet (CV), comprising a substantial proportion of vegetable plant-protein (VPs) derivatives, was formulated alongside two test diets. These were created by replacing graded levels of the VPs (20 and 40 %, respectively) with protein derived from Hi (Hi20 and Hi40, respectively). In all dietary treatments, the foregut, midgut, and hindgut exhibited general structural integrity, displaying intact intestinal folds, enterocytes, and no signs of inflammation. However, occasional mild lymphocytic infiltrations of the epithelium and lamina propria were observed in the pyloric caeca of the CV and Hi20 diets. The histological examination of the stomach revealed that the CV diet had induced moderately-severe lymphocytic gastritis, accompanied by a down-regulation of heat shock protein 70 (hsp70). Moderate lymphocytic infiltrations were observed in liver of fish fed with the CV diet, which correlated to a down-regulation of copper-zinc superoxide dismutase (Sod1). In the hindgut, an increase in the expression of Sod1, manganese superoxide dismutase (Sod2), and intestinal alkaline phosphatase (iap) and a decrease in the expression of hsp70 were observed in insect meal-containing diet relative to the CV. Overall, the present study showed that in sea bream a plant-protein based diet lacking in fishmeal negatively affects stomach mucosa, but HI included in a plant-protein-based diet helps in ameliorate histological distortion of tissue through specific correlated gene activations.