| Fishes of the family Hexagrammidae (greenling) are endemic to the North Pacific Ocean. Generally, they occur in nearshore benthic communities around the North Pacific Ocean from the Yellow Sea to Baja California, Mexico. Fat greenling, Hexagrammos otakii, is one of the most important commercial marine fish, distributed along Yellow Sea, Bohai Sea and East China Sea. However, owing to overfishing, pollution and deterioration of marine ecosystem, the genetic diversity of H. otakii is declined sharply in China. Greenlings of the genus Hexagrammos are a taxonomically interesting fish group that exhibits considerable morphological and ecological variability and hybridization between species. H. agrammus was one time classified as a separate genus(Agrammus) because this species exhibits a single lateral line, unlike all other members of the genera Hexagrammos. But presently except the work of molecular phylogeny of the hexagrammid fishes, little research has been done on genetic structure and genetic diversity for the greenling fishes. The article compared on biology characteristics and sequence comparison analysis of mtDNA and SSR between populations of H. otakii inhabiting the Bo Sea and Yellow Sea and Sea of Japan. Determination of population genetic structure provides essential information to underpin resource recovery and aid in delineating and monitoring populations for fishery management. For effective conservation and use of H. otakii, it is first necessary to obtain knowledge of its genetic background.Seven meristic characters, sixteen morphological characteristics were measured and analyzed by using the principle component analysis method, cluster analysis method, multivariate analysis method and one-way ANOVA among three H. otakii (160 individuals) populations collected from Dandong, Dalian and Yantai. The multivariate analysis and one-way ANOVA results showed that there were significant differences among the three populations in three to six morphological characteristics. The results of cluster analysis revealed that there were divergence among three populations, which were significant differences between Dandong population and Dalian, Yantai populations. Further, the Dandong population have dominance over the populations of Dalian and Yantai populations in length, weight and other biological characteristics. According to Mayr's 75% rule, there are some differentiations among the three populations of H. otakii in Northern Yellow Sea.Sequence variation of the first hypervariable segment of control region was analyzed among 11 populations of fat greenling H. otakii from the the Bo Sea and Yellow Sea and Sea of Japan. A total of 57 polymorphic sites was detected in the 438bp segment of the control region among 275 individuals of H. otakii, defining 125 haplotypes. Mean haplotype diversity and nucleotide diversity for the 11 populations were 0.965±0.006 and 0.007±0.004, respectively. The NJ (Neighbor-joining) tree based on all haplotypes showed a consecutive cluster in phylogeny level of H. otakii in studied areas. AMOVA analysis and pairwise FST revealed little but significant genetic differentiation among populations and between the north group (Dalian, Dandong, Yantai) and the south group (Rizhao-A, Rizhao-B, Qingdao, Ganyu, Rongcheng). The Zhaoyuan population lacked some main haplotypes, which may result from "founder effect". Neutrality tests and mismatch distribution revealed that H. otakii had undergone population expansion in late Pleistocene.By the way of cross-species amplification of microsatellite loci in the greenling fishes, this study got 7 markers (3 markers from H. decagrammus) will be of potential use in studies of genetic diversity population structure in H. otakii. These 7 markers are used to determine the number of alleles, the observed and expected heterozygosity across the 11 populations. The Zhaoyuan population departed from Hardy-Weinberg equilibrium, which was discarded from the following analysis. Pairwise FST revealed larger and most significant genetic differentiation among populations. The UPGMA tree based on DA genetic distance showed that group of Northern Yellow Sea populations (Dalian, Dandong, Yantai) and group of Haizhou Bay populations (Rizhao-B, Ganyu) had gather a cluster with high bootstrap supports as well as the model simulation results of STRUCTURE software. The two Japanese populations (Matsushima, Aomori) had limited gene flow with Chinese populations based on genetic distance.The scholars of home and abroad didn't have some research conclusion on phylogeny of the hexagrammid fishes in the morphology and molecule. We analysed COI, Cyt b,16S rRNA and D-loop fragments as well as Bayesian phylogenetic analysis of combined data from COI,Cyt b and 16S rRNA gene in 4 hexagrammid species (A. agrammus, H. otakii, H. octogrammus and H. lagocephalus) endemic to the coast of China. The results showed that A. agrammus was most closely associated with H. otakii supported by phylogenetic tree of COI, Cyt b, D-loop and combined data. So our conclusion were that A. agrammus and H. otakii were sister species, which belonged to sympatric speciation and latest speciation in fishes of the family Hexagrammidae.Based on mtDNA, SSR and biology characteristics data, significant genetic differentiation among populations were revealed. The inshore distribution of adults and larvae and the orientation of marine gyres/currents suggested that coastal barriers could mainly shape the population structure of H. otakii. Meanwhile, the breeding season differentiation may play a important role in shape the population structure. The fishery management of H. otakii may benefit from our result.
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