Morphometric correlation and regression of morphometrical 

traits and total  weight of 24-month-old 

Apostichopus japonicus(purple sea cucumber)

Abstract: Sea cucumber Apostichopus japonicus is one of the most commercially important mariculture species in China. In recent years, the market requirement and consumption of sea cucumbers have gradually increased. Therefore, the sea cucumber farming industry has been developping rapidly and become the pillar industry of coastal fishery economic in China, especially in the north China. Although the development of sea cucumber industry is rapid, several issues are demanding prompt solutions, such as lack of basic biology research, germplasm degradation, slow growth and reduced stress resistance of the existing varieties. Compared with other aquaculture species, special varieties with highquality economic characteristics are few, and the lack of improved varieties in sea cucumber restricts the development of farming industry. Therefore, germplasm creation has become one of the key points of sea cucumber farming research.

At present, new varieties of sea cucumber have been selected and bred successively with excellent economic characteristics, such as fast growth rate, high temperature resistance and disease resistance after great effort of breeding research institutes and production units for many years. It has laid the germplasm foundation for the further development of the sea cucumber industry in China. A. japonicus（purple sea cucumber） is a kind with special skin characteristic of sea cucumber. These highquality strains will lay the foundation for the diversification of sea cucumber species.

Accurate size measurements are important for the determination of growth performance for aquaculture. Most methods are used to estimate growth through length frequency data analysis, which depends on body measurements and biometric relationship. It is very difficult to measure the growth characteristics of sea cucumbers because of the difficulty in determining age by stress and dirty discharge, water spitting after leaving water, and body stretching and deformation. Precise measurements of body weight and body length in sea cucumber are challenging under different conditions. The body length of sea cucumbers can change considerably by elongation or contraction, and the body weight also varies considerably depending on the amount of water in respiratory trees and the intestinal content. Body size and body weight relationship is also important in determining the nutritional condition of the organisms. At the moment, there are no standard evaluation methods for accurate measurement of sea cucumber. In addition, compound indices that combine different biometric parameters can be applied to generate more precise biometric relationships. 

The study was conducted to explore morphometric correlation of morphometrical traits and body weight of 24monthold A. japonicus（purple sea cucumber）. 300 individuals were selected and relaxed by placement in containers with menthol of 0.2 mol·L-1 for 5 to 10 min. Then boby length(L), body width(W) and total weight(TW) were measured for each individual. When the body was relaxed and stretched, the camera was used to take pictures of the samples, and the data of body length and width were obtained by image calculation with camera measure software. Body length was measured from the center of the tentacle crown to the anus(curved length). Body width was measured dorsally at the widest point. Total weight was measured with a digital balance, accurate to 0.01 g. Indirect methods were used to estimate  the growth by use of compound index. A compound index that combined body length and body width to produce SLW index(L×W) and its transformation Le were utilized to obtain more precise biometric relationships. Lengthweight relationship was evaluated and the optimal fitting morphometrical traits and curve model were also selected by regression analysis.

Results showed that the coefficients of variability of growth characters were 31.52%66.85% and the highest and lowest character was body weight and SLW, respectively. The regression analysis results of the best fitting model of morphometrical traits and body weight were all power function model and Le was the best fitting morphological parameter for lengthweight relationship. Biometric relationship was useful to carry out transformations of body length to total weight. Body length and weight relationship have not been established for all sea cucumber species. First study applying a compound index on A. japonicus（purple sea cucumber） was accomplished. Body length, as well as body width, was also an important morphometrical trait. Therefore, it could be assumed that recalculated length from SLW might be useful to get accurate body length of sea cucumber. This study established indirect method for estimation of total weight in A. japonicus（purple sea cucumber）. Using estimated body length(Le) from the compound index SLW enhanced weight estimation accuracy in this study. The SLW compound index could reduce the variability of body length to homogenize the samples, facilitating the calculation of growth parameters by indirect method. 

A power model was used in the weight regression of A. japonicus（purple sea cucumber）, and a strong correlation was found between weight and SLW. The 24monthold A. japonicus（purple sea cucumber） grew allometrically with negative allometry tendency. It grew rapidly with better performance of morphological traits than body weight. The relationship among morphological traits at different development stages was unclear, neither the impacts of morphological variation on economically important traits at different stages. This study developed an algorithm to estimate A. japonicus（purple sea cucumber） weight by measuring its feature, including body length and body width. The indirect method and Leweight regression models in this study may be useful in growth evaluation of A. japonicus（purple sea cucumber）. The results of this paper provide theoretical guidance for growth and cultivation for A. japonicus（purple sea cucumber）.　

Keywords: Apostichopus japonicus（purple sea cucumber）; sizeweight relationship; morphometrical traits; total weight