| The mangrove clam(Polymesoda)it is a large bivalve shellfish endemic to the mangrove area.This study through the investigation and sampling survey.Analysis the distribution of the mangrove clam in Beihai,Guangxi province,distribution characteristics and environmental factors affecting the distribution;By comparison of Polymesoda erosa and Polymesoda expansa proportions of ulnar traits at different age stages,discriminate whether there is a fast growth.Using 16S rRNA,COI and Cytb genes to compare genetic differences between P.erosa and P.expansa.The main results are as follows:(1)There are 2 species of mangrove clam in the Beihai,including P.erosa and P.expansa,only distributed in the mangrove forest under the beach.(2)The distribution characteristics of P.erosa and P.expansa were basically the same,and there was no significant difference in the distribution density of the high tide area and middle tide area(P>0.05),but they were significantly higher than that in low tide area(P<0.05).The distribution density of P.erosa high,middle,and low tide areas were 13±4.387/m2,14.222±4.409/m2,and 5.444±4.720/m2 respectively.The distribution density of P.expansa in the high,middle and low tide areas were 10.333±3.640/m2,9.667±4.359/m2,and 3.889±3.515/m2 respectively.The distribution density of P.expansa is slightly lower than that of P.erosa.(3)The mandibles of P.erosa and P.expansa of different age stages have significant differences,and discriminate whether there is a fast growth.(4)Analysis of principal components of eight variables including salinity,pH,coarse sand,medium sand,fine sand,very fine sand,silt,and density to analysis,the main environmental factors affecting the P.erosa and P.expansa,environmental factors,salinity and medium sand are the main environmental factors affecting the distribution density of P.erosa density and variance of the two accounts for 70%of the total components.Salinity,medium sand,and coarse sand are the main environmental factors affecting the distribution density of P.expansa,and the variance of the three factors accounts for 69%of the total composition.(5)By comparing the 16S rRNA gene,COI gene,and Cytb gene sequences of P.erosa and P.expansa,in the 16S rRNA gene fragments of P.erosa and P.expansa,164 variation sites were detected in addition of the insertion site,including 161 simple information sites.In the nucleotide sequence of the COI gene,116 variatin sites(except insertion/deletion sites)were detected including 107 simple information sites.A total of 121 mutation sites(except insertion/deletion sites)were detected on the Cytb gene fragment including 115 simple information sites.Based on the genetic distance of 16S rRNA,COI and Cytb genes,the genetic distances between P.erosa and P.expansa was found,the genetic distance between species was 0.112-0.218,and the genetic distance between species was much larger that within the species.In the P.erosa and P.expansa COI and Cytb protein encoding gene fragments,CO amino acid fragment detected a total of 9 amino acid subsitution,Cytb amino acid fragment detected a total of 17 amino acid substitution.Most of the base substitutions of the COI and Cytb gene fragments occur at the third position of the codon.The second position is the most conservative,indicating that most of the mutations are synonymous mutations. |
