Analysis on the Effects of Morphological Traits of Lutraria sieboldii on Its Body Mass Trait

Dan GUO Jian CHEN Ziqin ZHAI Tongtong REN Jijin WANG Lirong BAI Dahui YU

Abstract [Objectives] This study was conducted to explore the relationship between morphological traits and body mass trait of Lutraria sieboldii.

[Methods]110 were randomly selected from 120 2nd-instar L. sieboldii collected from the Tieshangang area of Beihai， Guangxi， and 132 were randomly selected from 150 shellfish at the instar of 0.6. Their morphological traits were measured： shell length （SL）， shell height （SH）， shell width （SW）， anterior length （AL）， posterior length （PL）， nose length （NL） in closed shell state， and maximum open shell width （OS） between two shells in closed shell state， and the body mass trait BM was also measured. Statistical methods such as path analysis and multiple regression were used for data analysis， and the effects of these seven morphological traits on the body mass trait were studied， respectively. The correlation between the tested seven quantitative traits and one body mass trait was all positive， all reaching an extremely significant level （P<0.01）.

[Results] The body mass trait of the shellfish at the instar of 2 had the highest correlation coefficient with shell length （0.922）， that is， shell length had the greatest direct impact on the body mass trait; the path coefficient was 0.700; and the final multiple regression equation established was BM=-124.882+1.189SL+1.551 SH+1.035SW+0.119NL， and the total determination coefficient （R2） on body mass was 0.849. The body mass trait of the shellfish at the instar of 0.6 had the highest correlation coefficient with shell length （0.859）， that is， shell length had the greatest direct impact on the body mass trait; the path coefficient was 0.494; and the final multiple regression equation established was BM=-1.917+0.111SL +0.021NL+0.078SW+0.032OS， and the total determination coefficient （R2） on body mass was 0.828. The multivariate regression variance analysis showed that the regression between the morphological traits and body mass trait of the L. sieboldii at the instars of 2 and 0.6 reached an extremely significant level （P<0.01）.

[Conclusions]This study provides a scientific basis for the selection of broodstock in the selection and breeding of L. sieboldii.

Key words Lutraria sieboldi; Morphological traits; Correlation analysis; Multiple regression; Path analysis

Received： March 7， 2021  Accepted： May 1 2021

Supported by Guangxi Key R&D Program （2018AB52002）; National Key R&D Program of China （2018YFD0901406）; Natural Science Foundation of Guangxi （2018GXNSFAA138197， 2021GXNSFAA075008）; General Project of National Natural Science Foundation of China （31873042）; 2021 Key Cultivation Project of Guangxi Key Laboratory of Beibu Gulf Marine Biodiversity Conservation （2021ZB02）; The Basic Ability Improvement Project for Young and Middle-aged Teachers in Guangxi Universities （2018KY0612）.

Dan GUO （1994-）， female， P. R. China， master， devoted to research about aquaculture.

*Corresponding author. E-mail： bailirong1152@163.com.

Lutraria sieboldii， commonly known as Xiangbiluo or Baoluo， belongs to Lutraria of Mactridae in Veneroida of Heterodonta in Lamellibranchia. In China， it is mainly distributed in Zhejiang （Zhoushan）， Guangdong （Nanao， Taishan， Yangjiang， Zhanjiang）， Guangxi （Beihai， Weizhou）， Hainan （Qinglan Port， Xincun）， Taiwan and other sea areas. In foreign countries， it is mainly distributed in tropical and subtropical Japan （Honshu， Shikoku， Kyushu） and other Southeast Asian countries[1]. L. sieboldii is a kind of tidal flat shellfish with high economic value. It is rich in nutrients， tender and delicious in meat， and is deeply loved by consumers. In recent years， the scale of artificial breeding of L. sieboldii has gradually led to obvious degradation of germplasm resources. Therefore， the artificial selection of L. sieboldii can not only protect its germplasm resources in time， but also improve the economic benefits of farmers[2]. In the selective breeding of shellfish， body mass traits are one type of the most direct target traits in the breeding of improved varieties， and they are also a direct reflection of the production performance. Because there is certain correlation between morphological traits and body mass traits of shellfish， a few morphological traits may have major influences on body mass traits. Therefore， through correlation analysis and path analysis， the relationship between shellfish morphological traits （shell height， shell length， and shell width） and yield traits （body mass）， as well as the effect of morphological traits on yield traits， was clarified， so as to achieve the purpose of increasing yield through the direct selection of morphological traits[3].

In China， Liu et al.[4] first studied the effect of shellfish morphology on the body weight of Azumapecten farreri by path analysis and multiple regression analysis， and conducted breeding of shellfish on this basis. So far， this method has been widely used in the breeding of various sea shellfish species[5-7]. Wei et al.[8] found that shell width was the main trait affecting the body mass of Callista erycina. Li et al.[9] confirmed that shell length was the main trait affecting the body mass of the 1st-instar Scapharca subcrenata， shell length and shell width were the main traits affecting the body mass of the 2nd-instar larvae， and shell width was the main trait affecting the body mass of the 3rd-instar larvae. The research by Zou et al.[10] confirmed that shell length was the main trait affecting the body mass of L. sieboldii. Chen et al.[11] found that shell width was the main trait affecting the body mass of Coelomactra antiquata. Lin et al.[12] showed that shell width was the main trait affecting the body mass of six-month-old Pteria penguin， while shell height was the main trait affecting the body mass of eight-month-old P. penguin. Zheng et al.[13] conducted a study on 1-year-old Chlamys nobilis and found that shell height was the biggest factor affecting its body mass. Wang et al.[19] found that shell length and shell height were the most important morphological characteristics determining the soft body mass， shell mass and total mass of Dosinia laminat. However， there has been no report on the effects of the morphological traits of L. sieboldii at different ages on its body mass traits.

In this study， with L. sieboldii at the instars of 2 （natural population） and 0.6 （artificial breeding） collected from the Tieshangang area of the Beihai Sea in Guangxi as the objects of study， seven morphological traits （shell length， shell height， shell width， anterior length， posterior length， nose length， open shell width） and one body mass trait （living body weight） were measured， and path analysis and other methods were applied to analyze the correlation between morphological traits and the body mass trait， study the factors that directly or indirectly affected the mass of shellfish， and establish multiple regression equations between the morphological traits and the mass trait. This study provides a scientific basis for the selection of parent shellfish in the breeding of L. sieboldii.

Materials and Methods

Material sources

The tested 2nd-instar L. sieboldii came from a wild population in the Tieshangang area of the Beibu Gulf in Guangxi， and the L. sieboldii at the instar of 0.6 came from the first generation of this wild population. 110 live adults at the instar of 2 with intact shells and 132 juveniles at the instar of 0.6 were selected for the experiment.

Measurement methods

Absorbent paper was used to absorb the moisture on the surface of individual shells， which were then weighed with an electronic balance （accurate to 0.01 g） for the body mass trait （BM）， and measured with a vernier caliper （accurate to 0.01 mm） for the shell length （SL）， shell height （SH）， and shell width （SW）， anterior length （AL）， posterior length （PL）， nose length （NL） in closed shell state， and maximum open shell width （OS） between two shells in closed shell state （see Fig. 1 for measurement methods）.

Data analysis

Excel2010 software was used to sort out the measurement results of various morphological characteristics of L. sieboldii， and calculate the estimated values of various morphological parameters （average value， standard deviation and coefficient of variation）. Referring to the calculation methods of the path analysis of various phenotypic traits and body mass trait by Du et al.[14] and Yan et al.[15]， SPSS 24.0 software was used to analyze the data of the sorted quantitative traits and calculate correlation， path coefficients and determination coefficients of the various morphological traits with the body mass trait. The significance level was set to P<0.05， and the extremely significant level was set to P<0.0 and the multiple regression equations of morphological traits on the body mass trait were established[14，16].

Results and Analysis

Statistical analysis of morphological traits and the body mass trait

The phenotypic parameter statistics of L. sieboldii are shown in Table 1. The parameter units of different traits were different， so they could not be compared. The coefficient of variation （Coefficient of variation （CV） = Standard deviation （SD）/Mean） is a characteristic number that measures the degree of data variation， and it has no unit[13]. The calculation showed that the order of the coefficients of variation in shellfish at the instar of 0.6 ranked as NL>OS>BM>SH>PL>AL>SW>SL. The coefficient of variation of nose length was greater than those of morphological traits， and the range of coefficients of variation of morphological traits was 9.81%-66.95%. The order of the coefficients of variation in shellfish at the instar of 2 was NL>OS>BM>SW>AL>PL>SL>SH. The coefficient of variation of nose length was also greater than those of morphological traits， and the range of coefficients of variation of morphological traits was 7.28%-42.18%. The coefficients of variation of the morphological traits at the two shellfish ages were significantly greater than that of the body mass trait. The larger the coefficient of variation， the greater the potential for selection of corresponding traits[17]. Compared with the body mass trait， morphological traits had greater potential for selection. After the K-S normality test， all the traits obeyed the normal distribution， and the path analysis of the body mass trait could be carried out[14].

Correlation analysis between traits

According to Table  shell length with shell height， posterior length and the body mass trait， nose length and shell height with the body mass trait， and shell width with the body mass trait， were highly correlated （R>0.7）; anterior length with shell height， shell width and posterior length with the body mass trait， and open shell width with all other traits， were weakly correlated （R<0.4）; and the rest were moderately correlated （0.7>R>0.4）. There was an extremely significant positive correlation between all the quantitative traits of L. sieboldii （P<0.01）. The correlation between the quantitative traits of the 2nd-instar shellfish was -0.266-0.92 and the correlation coefficients of shell length and the body mass trait was the largest， while the correlation coefficients of nose length with the body mass trait and shell width were the smallest. From the analysis of correlation strength， shell length with shell height， posterior length and body mass trait， nose length and shell height with the body mass trait， and shell width with the body mass trait， were highly correlated （R>0.7）; anterior length with shell height， shell width and posterior length with the body mass trait， and open shell width with all other traits， were weakly correlated （R<0.4）; and the rest were moderately correlated （0.7>R>0.4）. The correlation between the quantitative traits of the shellfish at the instar of 0.6 was -0.073-0.861. The correlation coefficient between shell length and the body mass trait was the largest， and the correlation coefficient between nose length and shell height was the smallest. For the shellfish at the second instar， the correlation coefficient between shell length and the body mass trait was the largest， and the correlation coefficient between nose length and shell width was the smallest. Correlation analysis showed the closeness between two traits， and when there are multiple traits， in order to understand the direct and indirect effects of morphological traits of L. sieboldii on the body mass trait， it is necessary to perform multiple regression analysis and path analysis on each quantitative trait[18].

Multiple regression analysis of morphological traits on the body mass trait

According to the analysis results of partial regression coefficients and T test in Table 3， shell length， shell width， shell height， anterior length， posterior length， nose length and open shell width had extremely significant effects on the body mass trait （P<0.01）， indicating that there was an extremely significant linear relationship between the independent variables and the dependent variable. Taking shell morphology as the independent variables and body weight as the dependent variable， the stepwise regression analysis method was used to obtain the multiple regression equations of the morphology and body weight of L. sieboldii： instar of 0.6： BM=-1.917+0.111SL+0.021 NL+0.078SW+0.032OS， R2=0.828; instar of 2： BM=-124.882+1.189SL+1.551SH+1.035SW+0.119NL， R2=0.849. From the results of the analysis of variance of the multiple regression in Table 4， it can be seen that for the instar of 0.6， F=152.435， P=0.000， and for the instar of  F=132.343， P=0.000， that is， the regression between various morphological traits and body mass trait reached an extremely significant level （P< 0.01）. According to the prediction by the above equations， the differences between the estimated values and the actual observation values were not significant， indicating that the above equation can be applied to actual production[13]. In this study， the correlation index R2 of the morphological traits and the body mass trait of L. sieboldii was 0.828 and 0.849， respectively， less than 0.850， indicating that the main variables that affect the body mass trait of L. sieboldii include other factors besides the morphological traits， and it may also be related to soft body mass， adductor muscle mass and other factors[19-21].

Path analysis of morphological traits to the body mass trait

According to the correlation coefficients between morphological traits and the body mass trait of L. sieboldii， the path coefficients of L. sieboldii morphology and body mass were obtained by the linear stepwise regression method， as shown in Table 5 and Table 6. It can be seen from the data in Table 5 that the morphological trait that had the greatest direct effect on BM of the shellfish at the instar of 0.6 was SL （0.700）， and that with the smallest effect was OS （0.102）; and SH had the largest indirect effect on BM through SL （0.520）， while NL had the smallest indirect effect on BM through OS （0.005）. And the indirect effect of SH on BM was the largest （0.586）. From the data in Table 6， it can be seen that the morphological trait that had the greatest direct effect on the BM of the 2nd-instar shellfish was SL （0.494）， and that with the smallest effect was NL （0.089）; and SW had the greatest indirect effect on BM through SL （0.437）， while NL had the smallest indirect effect on BM through OS （-0.006）. And the indirect effect of SH on BM was the largest （0.599）.

The results are similar to the results of other economic shellfish research. Zhang et al.[22] studied Patinopecten yessoensis and found that shell length had the greatest direct impact on the body mass trait. Yang et al.[23] studied Cyclina sinensis and found that the most important factor affecting body mass traits was shell length. Xue et al.[24] studied different-month-old Sinonovacula constricta new variety "Shenzhe 1" and found that the traits had the largest direct impacts on body weight traits were shell length， shell width and shell height. Du et al.[25] found that shell length had the greatest direct effect on the wet mass of the 1st-instar Chlamys farreri， and shell height had the greatest direct effect on the wet mass of the 2nd-instar C. farreri. However， some research results are different. For example， Wei et al.[8] studied C. erycina and found that shell width had the greatest direct effects on body mass traits， and Wu et al.[18] studied Tapes dorsatus and found that shell width had the greatest direct effects on its body mass traits.

Analysis of the degree of determination of morphological traits on body mass trait

Table 7 and Table 8 show the determination coefficients of various morphological traits of L. sieboldii on the body weight trait. In the tables， the diagonal line lists the determination coefficient of each trait on the body mass trait， and the part above the diagonal line lists the indirect determination coefficients of two traits in pairs on the body mass trait. It can be seen from Table 7 that for the 2nd-instar shellfish， SL alone had the largest coefficient of determination on BM （0.490）， followed by SH （0.202）， while OS alone had the smallest coefficient of determination on BM （0.002）; and paired SL and SH showed the largest codetermination coefficient on BM （0.038）， while NL and OS showed the smallest value （0.002）. It can be seen from Table 8 that for the shellfish at the instar of 0.6， SL alone had the largest coefficient of determination on BM （0.248）， and NL alone had the smallest coefficient of determination on BM （-0.001）; and the two traits showing the largest codetermination coefficient on BM were SL and SH （0.244）， while SH and NL exhibited the smallest value （-0.001）. The results were basically consistent with the results of path analysis， and both results indicated that shell length was the main morphological trait that affected the body mass trait of L. sieboldii.

Fig. 2 shows the determination coefficients of the morphological traits of L. sieboldii on its yield trait. NL-NL， OS-OS， SH-SH， SL-SL and SW-SW represented the coefficients of determination of single independent variables on yield trait， and SL-SH， NL-OS， NL-SW， SH-NL， SH-SW， SL-NL， SL-OS， SL-SW， SW-NL and SW-OS represented the determination coefficients of two independent variables on yield trait. Among the determining effects of individual morphological traits on the BM of L. sieboldii， SL had the largest independent determining effects on the shellfish the instars of 0.6 and  and the corresponding coefficients of determination were 0.828 and 0.849， respectively.

Discussion and Conclusions

Shellfish have different morphologies， but there is a certain correlation between shellfish morphological traits and body mass traits. The yield of shellfish is controlled by multiple genes， and has close genetic links with other traits， and they influence each other[29]. Phenotypic traits as quantitative traits are the main content of shellfish genetic breeding research. A certain morphological trait may have a major impact on body mass traits. Therefore， to carry out the selection and breeding of L. sieboldii， starting with the determining effect of each morphological trait on the target trait can realize the screening of the best mass trait. Through the correlation analysis of shell type and growth index， the shell type closely related to economic traits can be screened out， which is of great significance in the genetic improvement， artificial breeding and breeding of shellfish[15]. In the process of selection and breeding， if body mass traits are used as the indexes of direct selection， large systematic errors may occur due to the interference of environmental factors， while identifying the main traits that affect body mass traits by the path analysis method and performing indirect selection can minimize the errors[27]. Therefore， correlation and path analysis are used to find out the relationship between shellfish morphological traits and yield traits， as well as the effects of morphological traits on yield traits， so as to find out the main morphological traits that affect body mass traits. Furthermore， the purpose of obtaining high-yield shellfish can be achieved through direct selection of morphological traits[3，13，25].

In shellfish breeding， shell morphology （shell length， shell height， shell width） and body mass （living body mass， soft body mass） are important measurement indicators for shellfish[23]. Therefore， clarify the relationship between morphological traits and body weight traits by path analysis and multiple regression analysis had very important practical significance for selective breeding. Path analysis can determine the correlation between traits， and split the correlation coefficient between traits into direct effects and indirect effects produced through other traits[30]. The main factor affecting the body mass trait of L. sieboldii at the instar of 0.6 was shell length， followed by shell width， while the main factor affecting the body mass trait of L. sieboldii at the instar of 2 was the shell length， followed by shell height. The coefficients of variation of shell morphology traits （shell height， shell length and shell width） were larger， and the coefficient of variation of the body weight trait was smaller. And the values of L. sieboldii at the instar of 0.6 were greater than those of the shellfish at the instar of 2. The coefficient of variation is the key reference basis for selection and breeding， and when the variation is large， the potential of selection is also greater， and the value of carrying out selective breeding is also higher. The reason for the above results might be that there was a transitional period from the growth of juvenile L. sieboldii to adult growth， or L. sieboldii grew faster in the early stage and slower in the later period， leading to large morphological variation[25-26]. The shell length and shell width increased the fastest in the early growth stage， and the shell length and shell height mainly increased in the later period. It is consistent with the result obtained by Lin et al.[12] that the maximum growth line of Pteria penguin before the 6-month age was mainly along the width of the shell， and when it developed to the age of 8 months， it gradually began to grow in the direction of shell height， that is， the growth rate in the direction of shell height was greater than the growth rate in the direction of shell width， and the maximum growth line started to change from the shell width direction to the shell height direction. It is also consistent with the result obtained by Zou et al.[10] that both shell mass and shell length traits could have a significant impact on the body mass traits of the selected population of L. sieboldii. The difference is that they chose shell length that is easy to measure as the preferred selection trait， and then strengthened the collaborative selection of shell mass. The biological characteristics of different shellfish are different， they all have their specific genetic genes， morphological characteristics， and the living habits， living environment， and growth stages of each species of shellfish can affect the differences of their morphological traits on body mass traits[8，11，18]. We compared the orders of the growth traits of L. sieboldii at different stages， and the families with fast growth in the early stage also had certain growth advantages in the middle and late stages. Therefore， it is possible to combine the characteristics of the stages during the growth of L. sieboldii to screen the traits in the juvenile stage and the adult stage， so as to screen out excellent individuals that can be used for genetic selection.

During artificial selection of L. sieboldii and its reservation for breeding， the selection of corresponding traits should be combined with different shell ages. When taking body mass traits as high-yield breeding targets， the shell length and shell width traits should be considered first in individuals at the instar of 0.6， and shell length and shell height traits should be considered first in individuals at the instar of 2.

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