Hot water crude extracts of green seaweed, Chaetomorpha linum, used as feed additives improve the growth, feed efficiency, and health of whiteleg shrimp, Litopenaeus vannamei postlarvae

The investigation was conducted to assess the impacts of hot water crude extracts of the green seaweed Chaetomorpha linum (HWCL) as a feed additive on the growth, feed efficiency , salinity stress tolerance, and bacterial resistance of the whiteleg shrimp Litopenaeus vannamei postlarvae (PL) . To initiate the experiment, shrimp PL with an average initial weight of 0.038 g, were assigned to different dietary groups. These groups received commercially available feed supplemented with varying concentrations of HWCL: 0 % (control), 0.3 %, 0.6 %, 0.9 %, and 1.2 %. A random set-up with three replicates in fifteen 500-L tanks was adopted with a stocking rate of 1000 PL/m 3 at water salinity of 15 ppt. After the 30-day growth trial, shrimp fed a diet supplemented with 0.6 % HWCL exhibited the most significant improvement in weight gain and feed conversion ratio (FCR) compared to the control and other groups. In addition, experimental shrimp were exposed to abrupt low (15 ppt to 0.5 ppt) and high (15 ppt to 50 ppt) salinity shocks for 150 min. Shrimp received diets supplemented with HWCL displayed significantly lower cumulative mortality rates compared to the control group. The remaining shrimp were then reared for another 12 days (individual weight ≥1.5 g) after which their health status was evaluated using an immersion challenge test. This test exposed the shrimp to pathogenic bacteria, Vibrio parahaemolyticus , at a concentration of 3.3 × 10 4 CFU/mL, for 14 days. The findings revealed that hemato-immunological component, including total hemocyte count, hyaline and granular cells, phenoloxidase , superoxide dismutase , and respiratory burst activities, were all significantly higher (p < 0.05) in shrimp fed HWCL diets compared to the control group, both before the challenge (Day 0) and 7 days after exposure to the harmful bacteria. This enhanced immune response correlated with significantly lower (p < 0.05) cumulative mortality rates in the HWCL-fed groups compared to the control at Day 14. Notably, the 0.6 % HWCL diet achieved the optimal balance between growth performance, FCR and improved stress tolerance against abrupt salinity changes and V. parahaemolyticus infection. Additionally, the highest tested concentration (1.2 % HWCL) yielded the greatest level of stress and pathogen resistance, but it also caused a significant reduction in growth rate and an increase in FCR. These results indicate that effective use of HWCL at 0.6 % dietary inclusion promotes growth, feed utilization efficiency, and the health of whiteleg shrimp postlarvae during the nursery phase.