Path analysis and allometric growth of morphological characters and body mass of juvenile Tachypleus tridentatus

The blood of the horseshoe crab, an ancient marine arthropod, is widely used in the production of tachypleus amebocyte lysate (TAL) for medical testing. The number of horseshoe crab resources in China has declined significantly in the last 30 years. The study of the relationship between morphological traits and body mass of Tachypleus tridentatus and the analysis of anisotropic growth will not only help to understand the growth characteristics of juvenile T. tridentatus, but also reflect the resource allocation strategy and help to conserve horseshoe crab resources. In this study, the cephalothorax length (X₁), cephalothorax width (X₂), opisthosoma length (X₃), opisthosoma width (X₄), telson length(X₅), total length (X₆) and body mass (Y) of 2-4 instars juvenile T. tridentatus were measured, correlation analysis and path analysis were used to explore the effect of morphological traits on body weight. The results showed that the morphological traits with the highest correlation coefficient (r) of body weight of 2-4 instars juvenile T. tridentatus were cephalothorax width (X₂). Path analysis showed that the cephalothorax width (X₂) had the most immediate impact on the body weight of 2 and 3 instars juvenile T. tridentatus, and the opisthosoma width (X₄) had the greatest direct impact on the body weight of 4 instars juvenile T. tridentatus. The determination analysis showed that the cephalothorax width (X₂) had the largest determination coefficient for the body weight of 2 and 3 instars juvenile T. tridentatus, and the opisthosoma width (X₄) had the largest determination coefficient for the body mass of 4 instars juvenile T. tridentatus. The determination analysis was consistent with the path analysis. The cephalothorax width and body weight of 2-4 instars T. tridentatus are processed logarithmically.The allometric growth equation was obtained: y=−1.095+2.746x, and the regression coefficient is 2.746(b > 1). This shows that the growth rate of juvenile horseshoe crab body mass is greater than the growth rate of cephalothorax width at this stage, and it is at the stage of positive growth. The present study shows that the morphological traits of seedling screening of T. tridentatus at different age stages are distinct, and the evaluation of the germplasm of the juvenile T. tridentatus should take into account its allometric growth pattern. The current study also provides a basis for exploring the relationship between growth and traits, structure and physiology of T. tridentatus.