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The gut microbiota plays an important role in a variety of physiological functions such as intestinal digestion, metabolic homeostasis, immune response, and responses to disease treatment. Whether there is a relationship between gut microbial communities and fatty acid (FA) profiles of Chinese mitten crab is unclear. Hence, we analyzed the relationship between FA profiles and the gut bacterial communities of six Chinese mitten crab (Eriocheir sinensis) populations from different lakes. The crabs were sampled from six different lakes in Jiangsu Province, China. The FA profiles of these crab populations were compared and clustered, and then used to determine the relationship between geographic location and FA composition. We also characterized the gut microbial communities of these crabs using 16S rRNA high-throughput gene sequencing. The FA profiles varied significantly (P < 0.05) between crabs from different geographical locations. A similar trend was also observed in the gut microbial communities, which also varied significantly based on their geographical origin (P < 0.05). Furthermore, alpha diversity, cluster analysis, and matching bacterial community structures with specific locations revealed patterns that significantly linked FA profiles to the gut microbiota. Further analysis of FA profiles and gut microbial community generated patterns that linked the two parameters. Hence, it was observed that the gut microbial community seems to contribute significantly to the FA composition of the Chinese mitten crab. However, further studies need to be conducted to investigate the interactions between gut microbial communities and the biochemical composition of the Chinese mitten crab, which will ultimately unravel the complexity of microbial ecosystems for potential applications in aquaculture and species conservation.

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tang 1,2 ✉ in order to identify candidate genes or loci associated with growth performance of the newly established common carp strain, Xinlong, we conducted a genome-wide association analysis using 2b-RAD technology on 123 individuals. We constructed two sets of libraries associated with growth-related parameters (weight, length, width and depth) measured at two different grow-out stages. Among the 413,059 SNPs identified using SOAP SNP calling, 147,131 were tested for GWAS after quality filtering. Finally, 39 overlapping SNPs, assigned to four genomic locations, were associated with growth traits in two stages. These loci were assigned to functional classes related to immune response, response to stress, neurogenesis, cholesterol metabolism and development, and proliferation and differentiation of cells. By overlapping results of Plink and EMMAX analyses, we identified three genes: TOX, PLK2 and CD163 (both methods P < 0.05). Our study results could be used for marker-assisted selection to further improve the growth of the Xinlong strain, and illustrate that largely different sets of genes drive the growth of carp in the early and late grow-out stages. Genome-wide association studies facilitate identification of single-nucleotide polymorphisms (SNPs) and genes associated with important economic traits. In particular, growth is one of the most economically important traits for the aquaculture industry. At the genomic level, genes and loci controlling this quantitative trait received a lot of interest in fish; for example growth rate in Atlantic salmon and rainbow trout 1-3 , head size in catfish and common carp 4,5 , etc. As a powerful statistical tool for connecting traits to their corresponding genes, genome-wide association study (GWAS) offers the possibility to analyze a massive amount of data. This allows identification of single nucleotide polymorphisms (SNP) or genes that may be related to important economic traits, or other traits of interest 6. However, GWAS employs methods that require genome-wide SNP data produced by genome re-sequencing, so it is relatively costly, which limits its applicability 7. Although the genechip used to capture the genome wide molecular markers has been established for some farmed animals (e.g. cattle and pigs), the application of GWAS in aquatic animals still faces higher costs and practical problems. To account for this, a simplified and cheaper genotyping method, 2b-RAD, was developed 8. This method is based on the sequencing of uniform DNA fragments produced by type IIB restriction endonuclease. Its effectiveness in associated analysis and genotyping has been established in several freshwater and marine fish species, including bighead carp (Hypophthalmichthys nobilis), Nile tilapia (Oreochromis niloticus), and yellowfin tuna (Thunnus albacares) 9-12. The common carp (Cyprinus carpio L.) is one of the oldest and most important farmed fish species, and the most widely distributed freshwater fish in the world 13,14. Ranking third among the most commercially important fish sp...