Black soldier fly (Hermetia illucens) larvae frass is emerging as a promising, sustainable, cost-effective ingredient for aquafeeds. Although its nutritional potential in tilapia diets has been investigated, its effects on fish health and immunity remain unclear. This study evaluated the effect of BSF frass on intestinal health and immune response in Nile tilapia (Oreochromis niloticus).
A 6 -week feeding trial was conducted with Nile tilapia (initial body weight : 120.74 ± 1.79 g) in tanks (300 L each ) in a RAS. Two diets were formulated based on the known nutritional requirements of Nile tilapia (Table 1 ): a control diet and one containing 30% BSF frass. Fish were distributed into six tanks of 20 fish each, with triplicate tanks per treatment, and fed to apparent satiation three times daily (10:00, 12:00, and 14:00 h) .
After the trial, light microscopy analysis of the distal intestine revealed that BSF frass inclusion did not significantly affect mucosal fold length (MFL), lamina propria width (LPW), intraepithelial lymphocyte abundance (IELs), or goblet cell density (GCD) compared with the control fed fish (Fig.1 ). However, e lectron microscopy analysis revealed a significant increase in microvilli density (MVD) and microvilli length (MVL) in BSF frass fed fish compared with the control (Fig.2) . Intestinal transcriptional analysis showed no significant changes in cytokines gene (il1β , tnfα , il10 , and tgfβ ), whereas hsp70 expression was significantly downregulated in the BSF frass group (data not shown ). Additionally, growth performance remained unchanged across the diets. Overall, these results suggest that 30% BSF frass can be included in Nile tilapia diets without adverse effects on intestinal health , immune response, or growth performance, supporting its potential as a viable feed ingredient.