Effects of the use of the probiotic Bacillus spp. on larvae of Cobia (Rachycentron canadum) raised in a water recirculation system
Cobia (Rachycentron canadum) is a marine fish that has good potential for its cultivation. Much research has shown the beneficial effects of the probiotic Bacillus spp. in aquaculture, however, there are no studies on the effects of probiotics on water recirculation systems (SARS). This work analyzes the effects of the probiotic Bacillus spp. in the performance of bijupira larvae raised in SRA. The experiment was carried out at Virginia Tech University (USA). The larvae were stored in two SARS independent of each other - probiotic treatment (TP) and control treatment (TC) - at a density of 15 larvae/L, for 26 days after hatching. One of the systems (TP) received the addition of commercial probiotic composed of B. subtilis, B. licheniformis and B. pumilus directly in the water and via live food, according to the manufacturer. The microbiology and water quality analyzes were performed daily by measuring physical and chemical parameters and plating in culture medium to count total Bacillus and Total Vibrio. At the end of the experiment, biometrics and total counting of the animals, resistance test to saline stress and larvae fixation for thymus immunohistochemistry were performed to evaluate the expression of multireceptor complexes of CD3 and CD4 antigens of T lymphocytes and cephalic kidney morphometry. Survival was similar and 14.9 ± 2.4% and 15.1 ± 0.9% for TP and CT respectively. The final weight was 170 ± 10 mg for PT and 160 ± 10 mg TC, with no statistical difference between treatments. In water microbiology, no significant differences were found between treatments in the total vibration count (TP: 3,366 ± 522 CFU/mL; TC: 4.964 ± 902 CFU/mL). In the total Bacillus count, the treatments differed statistically, with a higher concentration in TP by the addition of Bacillus spp. (901 ± 88 CFU/mL and 61 ± 24 CFU/mL). The salinity stress test demonstrated greater resistance to salt stress by the larvae of the probiotic treatment. Immunohistochemical analysis showed a higher expression of CD4 in the thymus in PT (TP: 43.7 ± 6.16% CD4 cells/mm2; TC: 25.4 ± 3.5% CD4 cells/mm2), which may indicate a better immune response against pathogens. Morphometric analysis of the cephalic kidney indicates that this organ is larger in TP (TP: 0.97 ± 0.33 mm; TC: 0.69 ± 0.22 mm), probably due to an increase in the amount of B lymphocytes. These results indicate that the probiotic used in a water recirculation system is effective in terms of immunostimulation and increases resistance to salt stress.