Aims: To reveal the effects of fish genotype, feeding habits and serum physiological index on the composition of gastrointestinal microbiota, eight fish species with four different feeding habits were investigated. Methods and Results: The V1 to V3 regions of 16S rRNA gene were analysed by high-throughput sequencing (454 platform) to compare the gut microbiota of different fish species. A total of 551 995 high-quality sequences with an average length of 463 bp were obtained from the 48 samples. No significant difference was observed among the detected sequences obtained from fishes with different feeding habits (One-way ANOVA, F = 1Á003, P = 0Á400), but the number of OTUs among different feeding habits was significantly different (One-way ANOVA, F = 7Á564, P < 0Á001). Additionally, significant correlations were detected between the fish genotype and microbial composition (partial Mantel test, all P values = 0Á001) in the stomach, foregut and hindgut. Moreover, different core intestinal microbiota was also noticed in the eight fish species with different feeding habits. Conclusions: Feeding habits and genotype clearly affected the gastrointestinal microbiota of fish. Moreover, the evolutionary process shaped the serum physiological indexes of fish. Significance and Impact of the Study: This study provided much important information for developing commercial fish feeds.

5-7 mm in length were used. The pressure was measured using the pressure dependence of the superconducting transition temperature of a built-in pressure sensor made of high-purity tin [23]. Experimental data were corrected for diamagnetism using Pascal's constants.Visible Absorption Spectroscopy under Hydrostatic Pressure: Full absorption spectra were recorded between 450 and 750 nm by using a Carl Zeiss PGS-2 spectrometer. The hydrostatic-pressure cell made of hardened beryllium bronze with NaCl as the pressure-transmitting medium operated in the pressure range 10 5 Pa < P < 3 GPa (accuracy ≈ ± 0.1 GPa). The pressure was measured using the pressure dependence of ruby [24].