The majority of these miRNAs aligned well with miRNAs from humans and zebrafish and was also conserved in most vertebrates. Our analysis showed that 92% of O. melastigma miRNAs identified in the present study have orthologs previously found in zebrafish. Thus, species in the same taxonomic group appear to express similar miRNAs. The specific functions and gene targets of these teleost miRNAs are not well understood. Our qRT-PCR analysis further identified the hypoxia-responsive miRNAs: let-7a & miR-122 ; let-7a & miR-9-3p and VE-822 1232416-25-9 miR2184 in hypoxiaexposed marine medaka. In mammals, miRNAs are of great importance in the regulation of many fundamental biological processes such as cell proliferation, differentiation, apoptosis, signal transduction and organ development. Several known classes of miRNAs have been predicted to regulate specific biological pathways. For example, the first known miRNAs, lin-4 and let-7, were found to play a major role in developmental timing. Highly expressed miRNAs are likely to have essential and broad regulatory functions. For example, mature let-7 regulates cell proliferation and differentiation and is also highly conserved across animal species. Let-7 is one of the most highly expressed across all tissues in our data. Thirteen members of the let-7 family were identified in O. melastigma, 8 of which were common to the male and female tissue investigated. All let-7 members share a similar seed region which regulates the interaction between miRNA and its target genes, so it is generally believed that let-7 family imposes similar biological functions among different species. There is ample reports that demonstrated the importance of let-7 in different biological functions of brain, liver and gonads. Sabrina et al reported that the introduction of let-7 caused the activation of Toll-like receptor 7, resulting in neurodegeneration in mouse’s brain. Also, let-7 was found to control glucose homeostasis and insulin sensitivity in liver. Furthermore, let-7 regulated ageing of testis stem cell niche in drosophila. Other highly expressed miRNAs identified in O. melastigma include miR-9, miR-21, miR-29, miR-122, miR-124, miR-143 and miR-202-5p. The orthologous miRNAs identified in the present study have diverse annotated regulatory functions. The brain-enriched miR-9 and miR-124 are both known to have a crucial role in neurogenesis and neuronal development, particularly in the regulation of neural differentiation, proliferation and cell migration. The expression of miR-122 was found to be liver-enriched. This is consistent with the results of previous studies and is consistent with its known important role in cholesterol and fatty acid metabolism. Recently, let-7, miR-122 and miR-143 have been shown to have tumor suppressive activity, and de-regulated let-7 expression has been associated with cancer.