Results of the present study demonstrated that the survival and growth performance of large yellow croaker larvae were significantly enhanced by supplementation of 40 mg/kg FA, which was consistent with previous findings of farm livestocks in lambs (Macías-Cruz et al. 2014; Valadez-García et al. 2021), heifers (Pena-Torres et al. 2021), weaned piglets (Wang et al. 2020), and fish in Nile tilapia (Dawood et al. 2020; Yu et al. 2020a, b), and zebrafish (Danio rerio) (Wen and Ushi 2017). These researches demonstrated that supplementation of dietary FA could promote the survival and growth performance of large yellow croaker larvae, which were probably due to its improvement in the activities of digestive enzyme, anti-oxidation capacity and lipid metabolism.
Improving the activity of digestive enzyme generally led to the advancement efficiency of nutrient digestion and absorption, and then enhance digestive system function of fish larvae (Izquierdo et al. 2000; Khoa et al. 2021). Previous studies have found that enhancing digestive enzyme activities can promote the survival and growth of fish larvae (Imentai et al. 2020), which was similar to the results of the present study. AKP, concentrated in BBM, is an important indicator of digestive and absorptive functions of the intestine (Yuan et al. 2020). In the present study, FA in diets not only improved the activity of trypsin (PS and IS) and lipase (PS), but also significantly improved activities of AKP (BBM), indicating the improvement of digest ability of large yellow croaker larvae. These results indicated that FA could improve activities of digestive enzymes of large yellow croaker larvae, which were consistent with the results in Nile tilapia (Yu et al. 2020b).
The antioxidant function of fish larvae is not mature, which made the larvae susceptible to the influence of external environment and resulted in slower growth (Birnie-Gauvin et al. 2017). Various kinds of stresses may trigger the production of reactive oxygen species (ROS), then cause oxidative injuries such as lipid peroxidation and DNA damage (Martínez-Álvarez et al. 2005). In order to cope with the stresses, antioxidant defenses system has been developed, containing T-SOD, CAT, and T-AOC (Martínez-Álvarez and Morales 2005). In the present study, activities of SOD and CAT in larvae fed diets with FA were significantly higher than the control group. The result was similar to the study in Nile tilapia (Yu et al. 2016, 2020a), which found that the supplementation of dietary FA could decrease the MDA content and increase activities of SOD and CAT. MDA is the biomarker of oxidative damage (Del et al. 2005). In the present study, MDA content in larvae fed diets with 20 and 40 mg/kg FA were significantly decreased compared to the control group, suggesting that FA reduced the oxidative damage of large yellow croaker larvae. Also, Maoka et al. (2008) reached a similar conclusion that the addition of dietary FA significantly reduced the hepatic MDA content of Red Sea Bream. Therefore, diets supplemented with 20 to 40 mg/kg FA could enhance the antioxidant capacity of large yellow croaker larvae by increasing activities of antioxidant enzymes and reducing the damage of oxygen free radicals.
Antioxidant capacity was positively correlated with reduced inflammation (Li et al. 2019; Mukherjee et al. 2013). Herbal extract have been shown a positive effect on the immunity of fish (Giri et al. 2019; Nootash et al. 2013; Wang et al. 2015; Zemheri-Navruz et al. 2019). The promotion of non-specific immune system not only can help fish to overcome infectious factors but also has an important preventative role (Galina et al. 2009; Punitha et al. 2008; Reverter et al. 2014). The immune function of FA had already been shown in rats (Katayama et al. 2017; Sadar et al. 2016; Zheng et al. 2019) and carp (Cyprinus carpio) (Ahmadifar et al. 2019). In the present study, FA in diets significantly increased activities of LZ, T-NOS, and NO content of large yellow croaker larvae. Results were similarly to the Nile tilapia, which significantly enhanced the activity of LZ and NO content (Yu et al. 2020a). The obtained result in this study indicated the potential effect of FA in large yellow croaker larvae on enhancement of immunity and inhibition of stress, similar to the previous study in Nile tilapia (Dawood et al. 2020).
Due to high energy consumption of larvae, promoting lipid catabolism would supply more energy and spare more protein to increase the growth performance (Li et al. 2016). According to previous researches, FA had shown its excellent function on improving lipid metabolism in mammals (Koh et al. 2017; Guo et al. 2019; Wang et al. 2020; Wang et al. 2019) and fish (Yu et al. 2016). In the present study, the supplementation of 40 mg/kg FA had significantly reduced TG content in visceral mass, which was similar to the results in the Nile tilapia (Yu et al. 2016). The results indicated that FA could reduce the lipid accumulation of fish (Cohen et al. 2011). The mRNA expression of scd1, fas and dgat2 were significantly down-regulated by supplementation of FA compared to the control group. Scd1, fas and dgat2 are crucial in the process of adipocyte synthesis, indicating that FA could decrease the lipogenesis of large yellow croaker larvae (Carobbio et al. 2013; Harris et al. 2011). Compared with the control group, supplementation of FA activated aco, cpt1 and hl genes thereby activating fatty acid oxidation and accelerating the lipid consumption process. The result was similar in mice with Ma et al. (2019). The mRNA expression of fabp3 and fabp11 in larvae fed diets with FA were markedly decreased compared to the control group. Results above showed that FA could reduce the lipogenesis and accumulation of large yellow croaker larvae, which was achieved by reducing the expression of lipogenesis-related genes, up-regulating the expression of lipid catabolism-related genes and down-regulating the relative expression of transport-related genes. Combined with the growth of large yellow croaker larvae in this study, we have reason to suspect that supplementation of FA in diets can promote the SR and SGR of large yellow croaker larvae through improving the lipid metabolism of larvae.