The blue crab, Callinectes sapidus Rathbun, 1896, is considered to be a luxury meal, especially in touristic cities. It contains more than 20 types of amino acids and provides all the needed amounts for human growth. This study describes the morphology and complex differentiation in the gonadosomatic index, morphological and ultrastructure features of the reproductive system, spermatogenesis, and spermatophores structure; this is due to the need to maintain natural and fishing stocks. Mature adult male crabs (carapace length 59 ± 7.12; width 126 ± 18.8 mm) were obtained from Abu-Qir Bay from November 2018 to October 2019 and transported alive in seawater to the laboratory. The reproductive system was dissected and weighed to the nearest 0.001g using the electronic balance, and the gonadosomatic index was subsequently calculated. The morphological analysis showed the developing testes with highly compacted seminiferous tubules. Using Periodic acid–Schiff stain, the spermatophore appeared with a zigzag-shaped wall that indicates its carbohydrate constituents. Each Spermatophore consisted of an inner spermatozoal mass embedded in a matrix, whose main components were secretions 1 and 2, and an outer thin acellular layer composed of secretions 3 and 4 from the anterior vas deferens (AVD). Secretions 5 and 6 (S5 and S6) also appeared with carbohydrate constituents using Mallory triple stain. The transverse section of the middle vas deferens (MVD) showed spermatophores with rod-shaped secretion S6 and granular secretions 7 (S7), forming a complex matrix between spermatophores. The secretion found in the MVD was granular, strongly acidophilic, and secreted by its highly columnar epithelium (S7). The ultrastructure showed that the testes were surrounded by a monolayer of myoid cells with an elongated nucleus, which also contained the following stages: spermatogonia, primary spermatocyte, secondary spermatocytes, and spermatids as well as spermatozoa. On the other hand, scanning electron microscope studies for fully formed spermatophore taken from the middle part of the vas deferens indicated that it is ellipsoidal in its outline with terminal stalk. Furthermore, the spermatophore was surrounded by a thick capsule of non-cellular substances and contained mature spermatozoa.

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