Morphology and Histological Observation of the Male Reproductive System in the Swimming Crab (Portunus trituberculatus)

The swimming crab (<i>Portunus trituberculatus</i>) is an economically important marine crab species in China. Understanding the structure and function of the male reproductive system in the swimming crab is crucial for increasing reproductive efficiency. Although studies on its reproductive biology have increased in recent years, the ultrastructure of the male copulatory system remains insufficiently explored. In this study, gonadal tissues [testes (T), anterior vas deferens (AVD), middle vas deferens (MVD), posterior vas deferens (PVD)] and copulatory organs [first gonopod (G1), second gonopod (G2), penis] of <i>P. trituberculatus</i> were sampled as experimental materials, and we systematically investigated the morphological and functional characteristics of male reproductive and copulatory organs in <i>P. trituberculatus</i> using anatomical, stereomicroscopic and scanning electron microscopic techniques. The results indicate that the male copulatory system comprises the G1, G2, and the penis. The G1 shows a tubular structure, with its endopodite bearing spines that anchor the female during copulation. The G2 is short, stout, solid and rod-shaped, with surface folds and blunt teeth, indicating its role in propelling the spermatophore within the G1. The penis is flexible and covered with elongated, rigid setae. Internally, the reproductive system displays a bilaterally symmetrical "H"-shaped architecture, consisting of the testes; anterior, middle, and posterior vas deferens; and the ejaculatory ducts. These segments have different structures and function in forming, storing, and transporting spermatophores. These findings provide important insights into the reproductive strategies and mating mechanisms of <i>P. trituberculatus</i>.