Plant proteins specially soya bean (SB) could be a promising source of protein in Aquatic animal feed, however, they are high in cellulose, which is difficult for fish and other monogastric animals to digest (Gatlin et al., 2007). Researchers are working on bioprocessing SB in other products such as soybean meal (SBM), soy protein isolate (SPI), fermented soybean meal (FSBM), and soybean protein concentrate (SPC) in shrimp diets in order to reduce ANFs (Abdul Kader et al., 2012; Gamboa-Delgado et al., 2013). In addition, fermentation of SB has been reported to increase its protein content (Teng et al., 2012), promote antibacterial and antioxidant activities (He et al., 2013; Akbari and Wu, 2015), and diminish immunoglobulin E immunological activity (Song et al., 2008).
The findings of this study indicate that 20% of FM can be substituted with FSBM without harming the health of shrimp. In addition, juvenile L. vannamei given 20% FBSM displayed enhanced growth performance and feed utilization, despite the fact that the amino acid composition of all experimental groups was identical. In addition, this improvement may be attributable to the improved lipid digestibility of FSBM (Refstie et al., 2005) and the reduction of anti-nutritional components during fermentation (Su et al., 2018). In contrast, the growth of shrimp fed a diet containing 40% FSBM was diminished. This decline may be attributable to the presence of non-digestible oligosaccharides, decreased protein digestibility, or a nutritional imbalance (Sharawy et al., 2016).
Our study's growth performance results were comparable to those of earlier research conducted on other crustacean species. According to the findings of Ding et al. (2015), the optimal growth performance of M. nipponense was achieved when 25% of the FM was substituted with FSM. Research on F. indicus suggests that replacing up to 28.57 percent of FM with FSM is significantly more economical (Sharawy et al., 2016). Shao et al. (2018) observed that a meal with a moderate FSM replacement level (20% of FM protein) efficiently boosted the growth of juvenile white shrimp and that a replacement level of up to 40% had no influence on shrimp growth performance.
In the present investigation, there were no statistically significant differences (P>0.05) in the moisture content of L. vannamei shrimp between diets. While the addition of S. cerevisiae increased the protein content of FSBM-fed groups significantly. The same rise in body lipid content was observed with increased dietary FSBM, which included more carbohydrates than FM (Makkar et al., 2007). According to Kaushik et al. (2004), one of the major factors leading to increased lipid retention is an increase in dietary plant protein, which is associated with an increase in hepatic lipogenic enzyme activity, imbalances in dietary amino acid content, and higher whole-body lipid levels in sea bass and salmonids. In addition, Sharawy et al. (2016) reported that there were statistically significant variations (P< 0.05) in the protein, dry matter, lipid, and ash content of shrimp fed different experimental diets compared to control diets containing 0.0% protein (FSBM). In our study, ash contents tended to decrease as FSBM levels increased from 0 to 40% of the meal.
Hemolymph metabolites serve as physiological, nutritional, and immunological stress indicators in crustaceans. In addition, it has been used to evaluate the nutritional health of shrimp, whose blood protein and glucose levels are very sensitive to the protein content of their diet (Rosas et al. 2001). In the current study, the inclusion of FSBM in the diet had a significant effect on the hemolymph total protein (TP) content, with the maximum TP concentration seen in groups fed 20% FSBM, followed by 30% FSBM. According to Shiu et al., (2015) the increased protein content of soya bean meal after fermentation may account for the observed results. On the other hand, the decreased TP concentration in 40% FSBM is due to the detrimental effect of high soya bean levels on the digestibility, absorption, and utilization of dietary protein (Gilani et al., 2012).
With its antibacterial effectiveness against bacterial infection, lysozyme activity is one of the most important indicators of shrimp immunity (Kaizu et al., 2011). The addition of S. cerevisiae to ferment SBM was also beneficial in regulating serum lysozyme activity. The results demonstrated a considerable increase in the lysozyme activity of 20% of FSBM-fed groups. The process involved in boosting the immune system of shrimp hinges on the protein recognition pattern of the circulating sugars which evoke the immune cells (Vargas-Albores and Yepiz-Plascencia 2000). It is assumed that the yeast harboring β-glucan effectively stimulated lysozyme synthesis. In contrast to what was expected, the lysozyme activity decreased as the concentration of FM-replacement FSBM increased. This was postulated as a result of the fatigue of lysozyme-producing cells from long-term exposure to the triggering agents, recommending the use of a low dose for a brief period of time for an effective response (El-Barbary et al., 2021; Babu et al., 2013; El Asely et al., 2011).
In addition to exogenous sources of reactive oxygen species (ROS), regular cellular metabolism generates electrons that can alter the membrane protein structure, lipids, cell division, and apoptosis signaling pathway (Redza-Dutordoir and Averill-Bates, 2016; Bauer and Bauer, 1999). Antioxidants' function in cells is to maintain balance and scavenge excess reactive oxygen species (ROS) to mitigate their corrosive effect (Kurutas, 2015).
In the present study, hepatopancreas CAT, SOD, Gpx, and GR activities increased significantly in the group fed 20%FSBM, indicating that the 20% substituted FSBM meal had a greater anti-oxidative effect than fish-fed FM and the other two concentrations. Ding et al. (2015) detected a drop in CAT, SOD, and GSH-PX activities with increased FSM content in the diet of Macrobrachium nipponense. The results obtained were almost identical to those reported by Ding et al. which suggested that the anti-oxidative capacity of shrimp was compromised by the substitution of fishmeal. Xu et al. (2008) found that fish CAT activity fell dramatically from 30% to 20% when fishmeal was substituted. Despite the fact that Daiyong et al. (2009) discovered that the CAT activity of shrimp was unaffected, the SOD activity declined dramatically when fishmeal was reduced from 25% to 20%. In addition to the enhanced flavonoid content created during soybean fermentation, tiny peptides, organic acids, and probiotics are also produced (Mukherjee et al., 2016). Saccharomyces cerevisiae produces vitamins and other metabolites that serve as exogenous antioxidant sources (Farid et al., 2019).
The hepatopancreatic MDA level of shrimp given 20% FSBM was much lower than that of shrimp fed the FM and other diets, demonstrating that the dietary replacement of FM with FSBM did not stimulate oxidative stress and was even successful at decreasing it. This could be attributable to the FSBM's high isoflavonoid content, which can neutralize free radicals and prevent lipid peroxidation (Yoon and Park, 2014).
The hepatopancreas is responsible for the generation and release of digestive enzymes, the absorption of nutrients, and the mobilization and transport of nutrients such as lipids, glycogen, minerals, and organic compounds to muscle and other tissues in response to growth and reproductive needs (Ceccaldi, 1989). The hepatopancreas secretes enormous quantities of digestive enzymes, such as amylases and proteases (Gamboa-delgado et al., 2003). Dietary content has a significant influence on digestive enzyme production and activity (Le Moullac et al., 1997; Guzman et al., 2001). In the present investigation, the substitution of fish meal with FSBM had a substantial effect on the activity of digestive enzymes; amylase activity was significantly higher in 40% FSBM and control diets than in other diets, indicating a higher carbohydrate content.
Interestingly, the digestive enzyme concentration in the intestine was substantially identical to that of the hepatopancreas, corroborating the findings of Córdova-Murueta et al (2003).
The hepatopancreas is the most essential digestive organ in shrimp. Histologically, it consists of four distinct cell types contained within blind-ending tubules. E-cells differentiate into R-cells (nutrient absorption and storage), F-cells (production of digestive enzymes), and B-cells (presumed to be secretory in function) at the apex of the tubules (Gopinath and Paul Raj, 2009).
In the present study, shrimp fed 30% partial fish meal replacement had significantly more R-cells than other groups (P < 0.05); this increase in R-cells, which are responsible for lipid storage in the hepatopancreas gland, may be indicative of an increase in energy reserve in the hepatopancreas as a result of the treatment. The B-cells are large cells responsible for enzyme storage, and this study revealed that their prevalence was significantly higher in the 40% partial fish meal replacement group than in the control group, but significantly lower in the 20% and 30% partial fish meal replacement groups (P < 0.05). In previous research, hypertrophied B-cells were identified in shrimp fed moderate dosages of the mycotoxin deoxynivalenol (DON), indicating oxidative stress in shrimp (Xie et al., 2018). Also, the increased B-cell prevalence in L. vannamei has been observed to increase at low salinities, suggesting that this is a response to the increased nutrient use required for higher osmoregulatory functions (Li et al., 2008). Similar to our findings, Romano et al (2015) concluded that while the prevalence of R-cells was significantly higher in shrimp-fed organic acids-blended diets, indicating greater energy reserves, the prevalence of B-cells, which are primarily responsible for the secretion of digestive enzymes, was significantly lower. The 30% and 40% partial fish meal replacement groups had substantially larger hepatopancreatic tubule diameters than the other groups (P < 0.05). The increase in hepatopancreatic tubule diameter may be correlated with the greater prevalence of R-cells and the resulting fat storage inside them (Johnston et al., 2003; Simon and James, 2007; Pourmozaffar et al., 2019). Which may be associated with the gross pathology picture of the hepatopancreas with the white fuzzy zone.