Analysis Of The Shell Characteristics Of Ruditapes Philippinarum In Different Geographical Regions Along The Coast Of Shandong Province

The Manila clam（Ruditapes philippinarum）is widely distributed,grows rapidly,and is highly adaptable.It is an important economic marine shellfish in China that has the largest R.philippinarum production in the world.Shandong Province is one of the most important breeding areas for the Manila clam^[1]with the main production areas in the Shandong Peninsula distributed in Jiaozhou Bay,Laizhou Bay,and Sanggou Bay.Shell traits are closely related to quality traits,which are often used as indicators to measure the economic value of R.philippinarum.However,there are no reports on the comparison of shell types,differences in shell traits,and mechanisms underlying the formation of shell types between these three coastal regions.In this study,R.philippinarum from three populations of the Jiaozhou Bay Hongdao sea area（HD）,Laizhou Bay Longwei sea area（LW）,and Sanggou Bay Chudao sea area（CD）in the Shandong peninsula were investigated and the differences in the shell traits and weights of clams from the three populations were analyzed.This was followed by a genetic and environmental analysis of the reasons for the differences in shell traits.The specific results are as follows:1.Analysis of the differences in shell and quality characters of R.philippinarum from three geographical populationsThe shell lengths,heights,and widths,together with single-layer shell thicknesses and wet weights and soft body wet weights of R.philippinarum were measured by geometric morphometry.The differences in shell characters,mass,and shape between the three geographical populations were compared and analyzed.The results showed that the morphology of clams from the HD population was significantly different from that of the other two populations,while the morphologies of clams in the LW and CD populations were similar.Clams in the LW population tended to be rounder,and the convexity index（shell width/shell height,SW/SH）was the highest.In contrast,clams from the HD population were flatter,and the elongation index（shell height/shell length,SH/SL）was the highest.In terms of shell thickness,the HD group had the thinnest shells and the LW group the thickest.There were no significant differences in the capacity index（shell width/shell length,SW/SL）.The highest correlations were seen between body weight and shell width,followed by shell height,while the correlation between the soft body wet weight and the shell length was the highest,followed by shell height.2.Effects of the breeding environment on the shape of the R.philippinarum shellWhen Philippine clams from the three geographical groups were transferred to the same sea environment for six months,it was found that the single-layer shell thicknesses of the three groups were similar.Principal component analysis（PCA）showed that the shell morphology of the three groups changed significantly from independent distributions during the initial stages of culture to overlapping distributions in the later culture stages,where it was difficult to distinguish between them.However,there was no significant change in the shell index between the three geographical groups,with the shell index（capacity index,elongation index,and convexity index）being essentially unchanged or showing minimal change.In the early stage of the ex situ culture,there was a strong correlation between the weight traits and single-layer shell thickness,while in the later stages of culture,the correlation between weight traits and shell length was highest.The results show that the thickness of the single-layer shell is greatly affected by the environment,while the shell shape index is hardly affected by environmental change.3.Transcriptome analysis of the mantle tissue of clams from the three geographical populations before and after ex situ cultureTranscriptome sequencing was performed on the mantle tissues of the three R.philippinarum populations before and after ex situ culture.A total of 811 509 760original sequences were obtained.After filtering the original sequencing data,118.58GB of clean data were obtained.The percentage of q30 bases was more than 92%.The quality of the transcriptome sequencing data was good.The assembled 230 267unigenes were compared with genes in six databases（NR,Swiss Prot,Pfam,COG,GO,and KEGG）,with an annotation rate of 24.01%.The statistics of genes expressed differentially between the three populations included 3010 differentially expressed genes in the CD population,4222 in the LW population,and 769 in the HD population.The number of up-regulated genes was found to be 1466 in the CD population,2262in the LW population,and 579 in the HD population,indicating that the numbers of down-regulated genes were greater than the up-regulated genes in the three populations.Functional enrichment using GO and KEGG identified nine target genes.It is speculated that the shell matrix protein may play a specific role in the regulation of shell type and that the mineralization gene may regulate the dimension of the convexity index（shell width/shell height,SW/SH）at the molecular level.It is speculated that VWA structure in Philippine clam larvae also inhibits the formation of the shell.The results of this study preliminarily reveal that shape changes of the R.philippinarum shell are influenced by both genetic and environmental factors.The shell type traits affecting the quality traits of the R.philippinarum shell grown in different geographical environments are also different.These results can be used for analyzing shape changes and their regulatory factors in the R.philippinarum shell,in-depth mining of functional genes related to shell formation,the protection of germplasm resources,and the breeding of improved clam varieties and carbon sink functions.