Morphological variation analysis of different geographical 

populations of Mactra chinensis

Abstract: From the perspective of breeding, the genetic background of parent shellfish without artificial system breeding or improvement is not clear, and there is a risk of inbreeding, which reduces the genetic diversity of the population and leads to the degradation of economic traits.Morphological traits and body mass traits are important basis for evaluating the genetic information of shellfish, however, there has been no report on the morphological differences of different geographical populations of Mactra chinensis. Therefore, it is necessary to conduct a comprehensive and systematic investigation and analysis of the current situation of Mactra chinensis population resources through morphological difference analysis, which can provide an important basis for the identification of geographical populations and protection and utilization of genetic resources of Mactra chinensis.The multivariate analysis method is suitable for the analysis of morphological differences of aquatic animal groups. The results can objectively reflect Morphological differences and have been widely used in shellfish. The multivariate morphometrics methods were used to explore the morpholobical variation of different geographical populations of Mactra chinensis along the coastal areas of Shandong Province. Researchers collected wild Mactra chinensis populations in Donggang（DG) in Rizhao ,Haiyang(HY) in Yantai , Huangdao（HD) in Qingdao,Wendeng（WD) in Weihai and Kenli（KL）in Dongying sea areas, respectively. Five Mactra chinensis populations were 2 years old, which were transported to the laboratory in batches at low temperature.A total of 50 Mactra chinensis were randomly selected from each group, morphological indexes were measured by vernier caliper, and body quality indexes were measured by electronic balance. The growth indexes of different groups of Mactra chinensis were calculated by Excel, including shell weight index and condition factor. Multivariate morphometric methods were used to analyze the morphological data of five wild populations of Mactra chinensis by variance analysis, principal component analysis, cluster analysis and discriminant analysis. The growth trait indices showed that WD population had the thickest shell, followed by HY population , KL population, DG population and HD population;there was no significant difference in ISW among WD population, KL population and HY population, and they were significantly higher than DG and HD populations ( P<0.05 ), HD population had the thinnest shell and the lowest condition factor and the condition factor of HY population was maximal, there was no significant difference in condition factor among 5 populations; the cofficient difference(C.D) between five geographical populations was 0.5151.245, less than 1.28, showing that the morphological differences among 5 groups were still intragroup differences, the morphological variation was characterized with undersubspecies; three principal components were constructed, of which the contributory ratio, from high to low, the principal component 1 was 33.71%, the loading value of body mass traits was the maximum, showing that the principal component 1 represented the influence of condition factor, shell thickness and width of shell on the shell shape; the principal component 2 was 27.16% , the shell height and the distance from the front end to the ventral margin had the greatest influence, with loading values of 0.700 and 0.686, respectively, it represented the influence of shell height and growth bias on shell shape,the principal component 3 was 14.42%, summing up to 75.29%, the three principal components explained 75.29% of the morphological differences among the five populations; cluster analysis of the average correction values of morphological indexes of five populations revealed that four populations clustered into two branches, HY population,WD population and KL population, DG population and HD population were grouped into one group respectively, it showed that morphological characters were rather similar between HY population and WD population, from which KL population was quite different; different geographical groups of Mactra chinensis were predicted and classified by discriminant function, four discriminant functions of five populations were established, the discriminant accuracy rate was 70.0%94.0%, with a total of 80.0%, the discriminant result was good, showing that the established discriminant function could be used to determine the attribution of a single Mactra chinensis individual in DG, HY, HD, WD and KL geographical groups.Studies showed that there were some differences in the morphology of five geographical populations of Mactra chinensis, the difference of population plumpness was related to seawater temperature and food abundance when samples were taken back. The size and shape of the shell, such as shell thickness, shell width, shell height and growth bias, determined the morphological differences among different geographic populations of Mactra chinensis, which was similar to the results of other shellfish studies. Mactra chinensis was strongly dependent on the environment, different ecological environments might have a certain impact on the morphology of Mactra chinensis, in the same or similar geographical or ecological environment, the morphology was consistent. In this study, KL population was quite different from the other populations, the morphological differences between HY population and WD population were small, and the morphology was the closest. the morphological differences between DG population and HD population were small, and the morphology was relatively close. This difference might be caused by geographical differences that resulted in different habitats. These environmental differences were likely to affect the growth and development speed of Mactra chinensis, which was reflected in the differences in shape measurement characteristics.The morphological characteristics of five geographical populations showed consistency with the geographical environment, which proved that the morphological differences of Mactra chinensis population were related to the geographical distance, the closer the distance, the more similar the ecological environment, and the smaller the morphological differences. WANG pointed out that the morphology of Monodonta labio was consistent in similar geographical or ecological environments, which was similar to the results of this study. However, in the study of CHEN and LIU  on the morphological differences of different populations of other shellfish, it was found that the clustering of population morphological indicators was inconsistent with the geographical location. The reason might be that the length of the planktonic period of different shellfish larvae was different, and the larvae with longer planktonic period would spread with the current. In addition, there were activities such as introduction of aquatic products and proliferation and release between different sea areas, which increased the probability of gene exchange between different groups and made it difficult to form effective geographical isolation. It also showed that the phenotypic variation of shellfish morphological characteristics was affected by genetic factors and environmental factors, morphological indicators could only reflect the combined effects of genetic and environmental factors among populations.

Keywords: Mactra chinensis; geographical population; morpholobical variation analysis