IMMUNONUTRITION IN WEANING DIETS: EFFECT OF Paecilomyces variotii (PEKILO®) ON GROWTH PERFORMANCE AND HEALTH OF GILTHEAD SEABREAM (Sparus aurata) LARVAE.

The replacement of fishmeal with sustainable microbial ingredients is a major challenge in marine larvi-culture, where nutritional quality and immune competence are critical during the weaning stage. This study evaluated the effects of dietary inclusion of Paecilomyces variotii (PEKILO®) mycoprotein in gilthead seabream (Sparus aurata) larvae. The filamentous fungus Paecilomyces variotii has a high nutritional value and with a range of functional bioactive components, including beta glucans and nucleotides. Four micro-diets were formulated to contain 0% (CTRL), 5% (5P), 10% (10P), or 15% (15P) P. variotii, corresponding to 0%, 33%, 66%, and 100% fishmeal replacement, respectively. Twenty-eight days old larvae were fed the experimental diets for 18 days, after which growth performance, survival, whole-body composition, histomorphological features, and immune responses were assessed, followed by a Vibrio anguillarum challenge. Results showed that fish fed up to 10P supported growth performance and survival, compared to CTRL group, whereas 15P inclusion reduced larval weight and post-challenge survival. Whole-body amino acid and fatty acid profiles reflected the increasing dietary level of P. variotii, with increased lysine and arginine, reduced glycine, higher n-6 fatty acids, and decreased DHA at the highest inclusion level. Histological analysis showed no intestinal alterations across treatments, while 15P reduced the incidence of moderate-to-severe hepatic vacuolization. Basal immune gene expression was only mildly affected by the diets, although larvae fed 5P and 10P showed increased expression of nfkβ1, il10, and mhcII. Following bacterial challenge, larvae receiving 5P and especially 10P diets exhibited stronger upregulation of immune-related genes and downregulation of casp3, indicating enhanced immune responsiveness. In contrast, larvae fed 15P showed poorer survival and no clear improvement in immune performance. Overall, moderate inclusion of P. variotii mycoprotein, particularly at 5%-10%, supported growth and improved immunocompetence in gilthead seabream larvae, whereas full fishmeal replacement compromised larval robustness. These results highlight the potential of P. variotii as a functional and sustainable ingredient in marine fish larval feeds when used at optimized inclusion levels.