The first complete mitochondrial genome of Goneplacidae （Decapoda：Brachyura）and its phylogenetic position among Brachyura

【Objective】To explore the evolutionary status of Goneplacidae in Brachyura from the mitochondrial genome level，so as to provide theoretical basis for better understanding the classification and evolutionary origin of Brachyura.【Method】Entricoplax vestita was selected as the representative species of Goneplacidae. The complete mitochondrial genome sequence of E.vestita was determined and analyzed. The complete mitochondrial genome sequence of the representative species was downloaded from GenBank. PhyloSuite was adopted to build a Brachyura Bayesian tree （BI）and a maximum likelihood tree（ML）simultaneously. 【Result】The total length of mitochondrial genome was 15716 bp，encoding 37 genes，including 13 protein coding genes，22 tRNA genes and 2 rRNA genes. The genome base composition was as follows：A（34.9%），T（37.7%），G（17.0%）and C（10.5%），showing obvious AT preference（72.6%）；AT skew and GC skew were negative values，which were -0.039 and -0.237，respectively. ATN was the starting codon of all protein-coding genes. Except for COII，COIII and Cyt b genes with T as incomplete termination codon，other genes were terminated with TAA or TAG. Except for the lack of DHU arm in the secondary structure of tRNA-Ser1 gene，other tRNA genes could form typical clover structure. The two rRNA genes were encoded by light chains，in which the length of 12S rRNA was 825 bp（between tRNA-Val and control interval），and the length of 16S rRNA was 1330 bp（between tRNALeu1 and tRNA-Val）. Based on the nucleotide sequences of 13 protein-coding genes，both Brachyura Bayes Tree（BI） and maximum likelihood tree（ML）showed that Goneplacidae and Oziidae were closely related，and the two were sister branches and clustered together with Panopeidae and Xanthidae. Majidae and Homolidae were non-monophyletic groups； the non-monophyletic properties of Calappoidea，Grapsoidea and Ocypodoidea were also strongly supported.【Conclusion】Goneplacidae shares the same gene rearrangements with that of ancestral genome of Brachyura；namely only tRNAF-ND5-H cluster changed to tRNA-H-F-ND5 order compared with Decapodamitochondial ancestral genome. Goneplacidae is most closely related to Oziidae，sharing the most recent common ancestor. Also，the results further verify the nonmonophyletic features of Majidae，Homolidae，Calappoidea，Grapsoidea and Ocypodoidea.