Effects of Single or Combined Administration of Dietary Synbiotic and Sodium Propionate on Humoral Immunity and Oxidative Defense, Digestive Enzymes and Growth Performances of African Cichlid (Labidochromis lividus) Challenged with Aeromonas hydrophila

The aim of the present study was to investigate the potential effects of dietary synbiotic (SYN) (Pediococcus acidilactici + Galactooligosaccharides; 10 g kg−1), sodium propionate (SP; 5, 10 and 20 g kg−1) and a combination of SYN + SP on the growth performance, humoral immunity, antioxidant responses and disease resistance against Aeromonas hydrophila of African cichlid (Labidochromis lividus) fingerlings (0.52 ± 0.05 g) in a feeding trial lasting 63 days. A completely randomized design was run with eight treatments, including 0 (control) and supplemented diets containing SYN + SP (e.g., 10 + 5, 10 + 10, 10 + 20, 0 + 5, 0 + 10, 0 + 20 and 10 + 10). The lowest feed conversion ratio value was observed in fish fed the 5 g kg−1-SP and 10 g kg−1-SYN (p < 0.05). The highest values of protein efficiency ratio and protein productive value were recorded in fish fed the 10 g kg−1-SYN (p < 0.05). Fish fed the 10 g kg−1-SYN diet had the highest activities of immunity (lysozyme, immunoglobulin) and antioxidant responses (glutathione peroxidase and superoxide dismutase) (p < 0.05). After 28 days post-challenge, the highest survival rate (57%) was recorded in the diet containing 10 g kg−1 SYN and 5 g kg−1 SP. The results indicated that the single administration of SYN or combined with SP, especially at the level of 5 g kg−1 of diet, enhanced the survival and growth performances, humoral immune response, antioxidant and digestive enzymes of African cichlid.

Neural activation in the olfactory epithelium of East African cichlid in response to odorant exposure

Fish use olfaction to gain a variety of information. To know what they receive and how they receive is important for understanding the species. However, studies on fish olfactory or pheromone receptors are still few and most of them are based on the neural response from a cultured cell. Here, we established a method to detect a biological-derived neural response from the olfactory epithelium of East African cichlid, the most diversified fish lineage, by c-fos in situ hybridization. We tested the response of microvillous neurons, which are expected to be dominated by V2R-expressing neurons, against several odorants. We showed microvillous neurons respond to amino acids the most whereas they do not respond to conjugated steroids. We next tested the response of V2R receptors. Although some subfamilies of V2R responded to amino acids, other did not respond which contrasts with the traditional hypothesis. Especially, one V2R subfamily responded to arginine. Not all the copies in this subfamily responded to arginine, which indicates the ligand differentiation in the cichlid-specifically expanded subfamily. Finally, we tested the response of putative pheromone receptor V1R to male urine. For this experiment, we established a new method to collect urine from cichlid. We showed two V1R receptors responded to male urine. Moreover, we showed that V1R2 receptor responded to 4-hydroxyphenyl acetate and lithocholic acid. This study is expected to provide a basis for the study on the olfaction of East African cichlids.