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Climate vulnerability and induced changes in physico-chemical properties of aquatic environment can bring impairment in metabolism, physiology and reproduction in teleost. Variation in environmental stimuli mainly acts on reproduction by interfering with steroidogenesis, gametogenesis and embryogenesis. The control on reproductive function in captivity is essential for the sustainability of aquaculture production. There are more than 3,000 teleost species across the globe having commercial importance; however, adequate quality and quantity of seed production have been the biggest bottleneck. Probiotics are widely used in aquaculture as a growth promoter, stress tolerance, pathogen inhibition, nutrient digestibility and metabolism, reproductive performance and gamete quality. As the gut microbiota exerts various effects on the intestinal milieu which influences distant organs and pathways, therefore it is considered to be a full-fledged endocrine organ. Researches on Gut-Brain-Gonad axis (GBG axis) and its importance on physiology and reproduction have already been highlighted for higher mammals; however, the study on fish physiology and reproduction is limited. While looking into the paucity of information, we have attempted to review the present status of microbiome and its interaction between the brain and gut. This review will address a process of the microbiome physiological mechanism involved in fish reproduction. The gut microbiota influences the BPG axis through a wide variety of compounds, including neuropeptides, neurotransmitter homologs and transmitters. Currently, research is being conducted to determine the precise process by which gut microbial composition influences brain function in fish. The gut-brain bidirectional interaction can influence brain biochemistry such as GABA, serotonin and tryptophan metabolites which play significant roles in CNS regulation. This review summarizes the fact, how microbes from gut, skin and other parts of the body influence fish reproduction through the Gut-Brain-Gonad axis.

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A study was conducted to evaluate the impact of acute sub-lethal toxicity of monocrotophos on behavioural, biochemical, haematological and histopathological changes in Heteropneustes fossilis (49.53±0.71 g). Probit analysis showed the 96 h LC 50 of monocrotophos for H. fossilis was 20 ppm. Followed by the LC 50 value, sub-lethal concentrations for acute exposure of monocrotophos were 3 ppm in T 1 , 6 ppm in T 2 and 8 ppm in T 3 for 72 h of the experimental period. The behavioural responses observed in treated fish were: erratic movement, imbalance in swimming, surfacing, and hyperactivities. A gradual reduction in total RBC count, haemoglobin, monocyte, and basophil contents were observed with an increased concentration of monocrotophos. On other hand, the total WBC count, neutrophil, basophil and blood ESR showed reversed trend (p<0.05). Total tissue protein content of gill, liver and kidney was altered and decreased significantly (p<0.05) in monocrotophos-treated fish. Tissue lipid peroxidation (LPO) and catalase activities in gill, liver and kidney were also altered after 24 h, 72 h and followed an increasing trend in exposed fish which differed significantly (p<0.05) from the control group. Compared to the control, significant changes were observed in the histopathological architecture of blood cells and gill tissue. The overall result showed that exposure to monocrotophos severely affects fish behaviour and physiology. Therefore the misuse of the chemical may be avoided to reduce the negative impact on aquatic animals.