Exploring polychaeta meal as a functional ingredient for improving European seabass intestinal health and oxidative status

In intensive aquaculture, fish are often exposed to stressors like handling, overcrowding, and confinement, which can compromise health and welfare. Nutrition plays a key role in boosting physiological fitness and stress resilience. Polychaete meal (PM), derived from Alitta virens , is emerging as a promising low-trophic alternative to fishmeal (FM) with potential functional properties. This study evaluated the effects of dietary PM inclusion at 2.5 %, 5 %, and 10 % in European seabass ( Dicentrarchus labrax ), focusing on intestinal health, non-specific immunity, and oxidative status under both baseline and acute stress conditions. PM diets significantly reduced intestinal immune cell infiltration, particularly at 10 % (PM10), with fewer granulocytes in the anterior intestine ( p = 0.009) and reduced lymphocyte presence in the rectum ( p = 0.004). Significant metabolic changes were also observed, including significantly increased plasma glucose in fish fed PM10 ( P = 0.017) and elevated lactate in the PM2.5 group ( P = 0.004), regardless of stress exposure. The PM5 diet significantly enhanced non-specific immune responses, increasing complement activity ( p = 0.026) and plasma immunoglobulin levels ( p = 0.023). Antioxidant defenses were also significantly improved across all PM groups, with higher GST activity observed in all groups ( p = 0.006) and elevated GSH content in PM2.5 and PM5 ( p = 0.040). Under acute stress, all groups displayed similar physiological responses, including significant increases in glucose, lactate, and cortisol, as well as altered innate immune markers, without significant differences among diets. These findings suggest that PM can promote health and immune status in European seabass under optimal conditions. Further research is warranted to assess its functional benefits under more challenging aquaculture scenarios. • Dietary inclusion of increasing PM levels did not affect acute stress response. • PM10 reduced leukocyte infiltration in the anterior intestine and rectum. • PM5 enhanced basal non-specific immunity and glutathione-dependent defenses. • PM2.5 and PM10 led to alterations in lactate and glucose turnover, respectively.