Abstract:The seminal receptacle of cephalopods plays a key role in its reproduction. The number and location of seminal receptacles of different cephalopod species are different, so the seminal vesicles of cephalopods are of interest to researchers, similar to the sperm that female cephalopods store. Sepioteuthis lessoniana had an oval milky white seminal receptacle located on the buccal membrane. In this study, histological and transmission electron microscopy techniques were used to analyze the ultrastructural characteristics of the seminal receptacle of S. lessoniana for the first time. The results showed that the seminal receptacle was composed of glandular parietal tissue, a sperm storage bulb, and a central lumen. The cyst wall tissue was composed of epithelial cells, muscle tissue, and connective tissue. Epithelial cells could be identified by the visible round nucleus in the center of the cell. The demarcation between the monolayer epithelial cells and the connective tissue was obvious but the boundary between the muscle tissue and the connective tissue was not obvious. The muscle tissue was mainly smooth muscle, with a thickness of approximately 140 μm to 240 μm. There were many sperm storage bulbs, most of which were irregular in shape and a few are round or oval with diameters, ranging from 180 μm to 300 μm, which was the main part of the seminal receptacle. Two or more sperm storage bulbs joined to form a central lumen, and connective tissue and muscle tissue were scattered around the sperm storage bulb and the central lumen. A large number of sperm were present in the sperm storage bulb and central lumen, and the acrosome and flagella of the sperm were stained dark blue and red, respectively. The sperm storage bulb and central lumen were composed of monolayer epithelial cells with round or elliptical nuclei. The cytoplasm contained a variety of organelles, including endoplasmic reticulum, mitochondria, and Golgi complexes. The endoplasmic reticulum could be divided into rough endoplasmic reticulum and smooth endoplasmic reticulum. The number of mitochondria was also higher. The elliptical Golgi complex was composed of multilayered, arched flat sacs and secretory vesicles, which release a large number of secretory granules after rupture. In addition, a large number of vesicles were found, which were filled with secretory granules. There were cilia outside the epithelial cells, and many sperms could be seen in the sperm storage bulb and central lumen. The sperm were closely arranged and orderly, and the head of the sperm is directed toward the cilia. The sperm of S. lessoniana had a slender head, which was composed of a dome–shaped acrosome and a slender cylindrical sperm nucleus. The mitochondrial spur was located at the posterior end of the sperm nucleus, and the tail was a slender flagellum. The results showed that the seminal receptacle can store sperm, the epithelial cells of the sperm storage bulb can secrete sperm, and the secreted substances can attract sperm, which plays a role in the process of directing sperm into the seminal receptacle.
