Aims A study was conducted to compare the intestinal microbial compositions of two fish species with similar feeding strategy; paddlefish (Polyodon spathala) and bighead carp (Aristichthys nobilis) reared in the same pond. Methods and Results Age‐0 paddlefish and bighead carp with mean average body lengths of 43·39 ± 2·78 and 19·33 ± 3·68 cm, respectively, were reared with natural prey items in the same pond (20 m²). After 30 days of rearing, the intestinal microbiota of the two fish species was assessed by pyrosequencing of 16S rRNA genes. Interestingly, deviations were observed in the microbial communities of the two fish species according to the alpha‐ and beta‐diversity measurements and detrended correspondence analysis (DCA). Shannon diversity (P = 0·015) and Pielou.evenness (P = 0·035) revealed significant lower diversity of the intestinal microbiota of paddlefish. Moreover, different core intestinal microbiota was noticed in the two fish species. Proteobacteria (57·3%), Firmicutes (11·9%), Fusobacteria (8·9%), Planctomycetes (7·3%), Actinobacteria (6·0%) and Verrucomicrobia (3·2%) were detected in bighead carp, while the dominant phyla in paddlefish intestines were Bacteroidetes (37·0%), Fusobacteria (35·1%), Firmicutes (14·8%) and Proteobacteria (12·6%). Conclusions Our results revealed that the intestinal microbiota differed between paddlefish and bighead carp reared in the same pond when fed similar nature food. The potential host factors, such as the genetic background, gut histology and physiology are assumed to be involved in the intestinal bacterial compositions. Significance and Impact of the Study Considering the similar feeding strategy of paddlefish and bighead carp, this study presents basic knowledge for evaluation of the importance of host factors (genetic background and gut anatomy) on intestinal microbial composition.

Largemouth bronze gudgeon (Coreius guichenoti) is of economic importance in China, distributed in upstream regions of the Yangtze River in China. But it has recently dramatically declined and is close to elimination. However, there is little knowing about the character of its intestinal microbiota. This study was conducted to elucidate the intestinal microbiota of wild largemouth bronze gudgeon with different body weight and gender. Thirty wild largemouth bronze gudgeon were measured for body length and body weight, and identified for male and female according to gonadal development, and thereafter the intestinal microbiota’s were assessed by MiSeq sequencing of 16S rRNA genes. The results revealed that phyla Proteobacteria and Tenericutes were dominant in wild largemouth bronze gudgeon intestine independent of the body weight. Shannon’s and Inverse Simpson’s diversity indexes were significant (P < 0.05) different between male and female fish. The phylum profile in the intestine of male fish revealed that phylum Proteobacteria was dominant, in contrast to female fish where five phyla Tenericutes, Proteobacteria, Firmicutes, Fusobacteria and Spirochaetes were dominant. The genus profile revealed that genera Shewanella and Unclassified bacteria were dominant in male fish, while genus Mycoplasma was dominant in female fish. Our results revealed that the intestinal microbial community of wild largemouth bronze gudgeon was dominated by the phyla Proteobacteria and Tenericutes regardless of the different body weight, but the communities are significant different between male and female fish. These results provide a theoretical basis to understand the biological mechanisms relevant to the protection of the endangered fish species.

Bighead carps (Aristichthys nobilis) were divided into four groups with different feeding strategies: group A, nature live food only (fertiliser only, 200 g urea + 160 g ethylamine phosphate + 250 g Huangjintai bio-fertiliser); group B, nature live food + 1/2 formulated feed; group C, nature live food + formulated feed; and group D, formulated feed only. The intestinal microbiomes of the different groups were compared through the Illumina MiSeq sequencing of the bacterial 16S rRNA gene. The specific growth rate (SGR), survival and blood biochemical factors of the fish were also investigated. Results showed that feeding treatment influenced the intestinal communities in the fish. In specific, more bacterial phyla dominated in groups A and B (phyla Bacteroidetes, Fusobacteria, Firmicutes and Proteobacteria in group A, phyla Proteobacteria and Fusobacteria in group B) than in groups C and D (phylum Proteobacteria). The diversity was also lower in groups C and D than in groups A and B. Unweighted pair-group method analysis revealed a clear difference in intestinal microbiota among the different feeding treatments. No difference in survival rate was found among the treatment groups, but the SGR was significantly higher (P < 0.01) in groups B, C and D than in group A. Functional analysis showed that the intestinal bacteria correlated with fish glucose metabolism in group A but with lipid metabolic activity in groups B, C and D. In summary, the intestinal microbiomes and their potential functions vary in bighead carp under different feeding treatments. This study provides new insights into the gut microbiomes of filter-feeding and formulated diet-fed fish.

This study was conducted to determine the optimal temperature for Juvenile Coreius guichenoti, which is critically endangered in China. Test fish with an initial body weight range of 17.60-18.78 g were reared in 5 independent recirculating aquaculture systems (RAS) with three tanks (50 fish/tank) for 8 weeks. The water temperature of the RAS was set at 14 °C, 18 °C, 22 °C, 26 °C, and 30 °C, respectively. At conclusion of the feeding trial, the effects of water temperature on C. guichenoti were analyzed based on survival, growth performance, digestive enzymes activities, and antioxidant capacity. The results showed that the fish survived in all temperature groups. The fish in the groups (26 °C and 30 °C) had higher values of body length, body weight, specific growth rate, weight gain rate, and daily gain than those in the group (22 °C). The lowest values of growth performance were observed in the groups (14 °C and 18 °C) among the 5 groups. The lowest values of feed conversion ratio (FCR) were observed in the group (26 °C). Regression analysis identified the minimum FCR at 26.55 °C. The feeding rates (FR) were increased gradually with the increase of temperature. Regression analysis identified the maximum FR at 28.33 °C. The activities of protease and lipase in the gut were higher than those in the liver, while amylase activity was lower in the gut than that in the liver. There were no significant differences in activities of protease and lipase among different parts of the intestinal tract at the temperatures range from 18 to 26 °C. When the temperature was out the optimal range, C. guichenoti would be under chronic stress, resulting in decreased lysozyme activity and cortisol concentration. Inconclusion, the optimal temperature for juvenile C. guichenoti is 26.55-28.33 °C based on the maximal growth performance and feed utilization.