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Written by the Editorial Team of Aquaculture Magazine
There is evidence that dietary additives can stimulate the shrimp immune system, but few studies have focused on the initial developmental stages. Findings suggest that tailored diets including health-promoting additives as vitamins C and E and β-glucans may address some of the larviculture problems and may contribute to the success of whiteleg shrimp farming in the long term, affecting the downstream production of high-quality juveniles and adults.
The whiteleg shrimp (Penaeus vannamei) is currently the most representative animal species in aquaculture, constituting in 2020 a share of 4.7% in global production. To meet the market demands, whiteleg shrimp larvae and post-larvae (PL) yields in hatcheries has increased intensively. How ever, problems in larviculture can have an enormous impact on shrimp performance in the long-term, affecting the downstream production of high-quality juveniles and adults.
Initial developmental stages are frequently associated with sub-optimal growth, high size dispersion, and low survival due to cannibalism and reduced disease resistance to opportunistic pathogens.
The latter results from a high dependence on optimal zootechnical conditions and nutrition, as shrimp lack an adaptive immune response and depend uniquely on their innate immune system to maintain a good health status and avoid pathogenic outbreaks that can result in disastrous consequences to production and significant economic losses.
Therefore, innovative nutritional solutions that enhance development and resistance to stress and pathogenic factors during these critical stages and thus improve shrimp quality in posterior phases of production have tremendous potential to reinforce the success of shrimp farming.
Below are the results of a study aimed to evaluate the effects of several health promoting nutrients/additives (i.e., vitamins C and E, β-glucans, taurine, and methionine) supplemented in inert microdiets on the growth performance and health status of whiteleg shrimp post-larvae.
Materials and methods
Four experimental microdiets were evaluated in triplicates. A positive control diet (PC) was formulated to meet the nutritional requirements of whiteleg shrimp post-larvae, containing 515 g kg−1 of SPAROS proprietary marine protein mix, 160 g kg−1 of SPAROS proprietary plant protein mix, 103 g kg−1 of fish protein hydrolysate, 19 g kg−1 of fish oil, 28 g kg−1 of marine phospholipids, and 57 g k−1 of SPAROS proprietary vitamins and minerals premix.
On the remaining treatments, three experimental variants based on the PC were used, differing only in the ingredient formulation by the following:
(1) decreasing inclusion levels of the vitamins and minerals premix by 7 g kg−1 to reduce vitamin C and E contents in the negative control diet (NC);
(2) supplementing 5 g kg−1 of taurine and 10 g kg−1 of methionine to increase the levels of both molecules in the T + M diet; and supplementing the PC diet with 1 g kg−1 of Saccharomyces cerevisiae β-(1, 3)/(1,6)-glucans (BG).
Whiteleg shrimp post-larvae (PL16), originated from Blue Genetics (La Paz, Mexico), were reared for 18 days at Riasearch Lda facilities (Murtosa, Portugal). Shrimp were randomly distributed to 12 tanks with approximately 50 L that were part of a clear water recirculating system.
Each tank was stocked with 200 individuals averaging 9 mg of wet weight. For the oxidative stress and immunity-related biomarkers the sample preparation consisted in a total of 40 whole whiteleg shrimp post-larvae from each tank sampled at end of the trial were weighed and homogenized in quadruple groups of 10 individuals for oxidative stress and immune parameters analysis.
The catalase (CAT), lipid peroxidation (LPO), and total glutathione (tGSH) activities as well as total proteins content were determined in the homogenized samples.
Results
Growth performance
No significant differences in growth performance and survival were observed among dietary treatments. Final wet weight averaged around 100 mg, relative growth rate (RGR) values 15% day−1, feed conversion ratio (FCR) was close to 1, and survival ranged between 86 to 88% for all treatments (Table 1).
Oxidative stress and immune status related biomarkers
Regarding the oxidative stress parameters measured, CAT levels were similar, with no significant differences being detected across treatments; LPO levels were significantly lower in shrimp PL fed the BG dietary treatment than those fed the PC diet, with no significant differences between the remaining treatments; tGSH levels were significantly higher in shrimp PL fed the BG treatment than in their counterparts fed the NC diet, with no significant differences between the remaining treatments.
As for the immune condition, no significant differences between treatments were observed regarding the parameters measured (Table 2).
Gene expression analysis
The normalized relative mRNA expression of the PvHm117 crustin P gene decreased significantly in shrimp PL fed the NC diet compared to those fed the T + M and BG dietary treatments. Similarly, the penaeidin 3a mRNA expression level decreased significantly in shrimp PL fed the NC diet compared to their counterparts fed the PC and BG dietary treatments.
Hemocyanin transcripts increased significantly in shrimp PL fed the NC diet compared to PL fed the T + M dietary treatment. As for the normalized relative mRNA expression of the remaining genes, no significant differences between treatments were observed (Table 3).
Discussion
This study aimed at evaluating the potential health-promoting effects of including several dietary supplements in inert microdiets for white leg shrimp PL. Vitamin C and E, methionine, taurine, and β-glucans were selected for this purpose since their potential ability to enhance the health status of whiteleg shrimp in the initial stages of development is still promising but yet to be experimentally validated.
A control diet formulated to fulfill the nutritional requirements of whiteleg shrimp PL was used as positive control, and the remaining experimental diets were based on it, differing only in the reduction or addition of the previously mentioned nutrients. In overall, the formulation changes in the diets did not compromise their adequacy, as good growth performances and survival results were obtained in all experimental treatments, also revealing that good zootechnical conditions were maintained during the trial.
Growth results were similar to those reported by Wang et al. (2017) when using graded levels of Schizochytrium meal and as a replacement of fish oil in practical diets for whiteleg shrimp PL, but survival results were considerably inferior in those studies (40.3 – 44.5% and 42.7 – 45.6%, respectively) than in the current trial.
Increasing the vitamin C and E supplementation levels in the PC dietary treatment did not produce any changes in growth performance and survival when compared with the NC, suggesting that the levels of these vitamins present in the NC diet still allowed the shrimp post-larvae to maintain an adequate development.
Like other vitamins, vitamin C and E are essential nutrients, as animals are unable to synthesize sufficient amounts to meet their physiological needs, and a deficient supply in the diet often results in poor growth, possibly leading to severe health issues and even compromising survival (Dawood et al., 2018; Nutrient Requirements of Fish and Shrimp; National Academy Press, 2011).
Additionally, no changes were verified in the activity levels among most of the immune and antioxidant parameters measured in this study when increasing the levels of these vitamins in the diets.
Inert diets with adequate levels of vitamins C and E can be vital during critical stages of production, particularly in farms where a nursery system is employed (intermediate step between the early PL stage and the grow out phase), in which PL are kept at extremely high stocking densities that can induce stress and vulnerability to opportunistic pathogens (Mishra et al., 2008).
Lipid peroxidation levels dropped significantly in shrimp PL fed the BG dietary treatment compared to those fed the PC diet, suggesting that β-glucans improved the antioxidant mechanisms of the animals.
Conclusions
In conclusion, the results obtained in this study suggest that although no improvements in growth performances and survival were observed at the end of the experimental period, all dietary additives tested have the potential to add value to inert microdiets for whiteleg shrimp PL.
Benefits to the antioxidant capacity and robustness of the shrimp PL were clearer when the vitamin C and E levels were higher than those used in the NC, similar to those used in the PC. However, the control diet can be considered a premium option, and it should be expected that the positive effects provided by these supplements are augmented when incorporated into more economical alternatives.
Amongst the additives tested, the inclusion of β-glucans in the diets seems to be the most promising, as it reduced lipid peroxidation in the shrimp PL even when compared to a high-quality control diet.
When compared to the NC, the interaction between the supplementation of β-glucans and higher levels of vitamins C and E also seems beneficial to the antioxidant capacity of white leg shrimp PL.
This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “HEALTH-PROMOTING ADDITIVES SUPPLEMENTED IN INERT MICRODIETS FOR WHITELEG SHRIMP (PENAEUS VANNAMEI) POST-LARVAE: EFFECTS ON GROWTH, SURVIVAL, AND HEALTH STATUS” developed by: ANDRÉ BARRETO – Riasearch Lda, CIIMAR, ICBAS-UP, DIOGO PEIXOTO – CIIMAR, ICBAS-UP, CARLOS FAJARDO – CIIMAR, Universidad de Cádiz, WILSON PINTO – Sparos Lda. The original article was published, including tables and figures, on FEBRUARY, 2023, through ANIMALS. The full version can be accessed online through this DOI 10.3390/ani13040726.
