Organogenesis and subsequent development of the genital organs in female and male Pacific white shrimp Penaeus (Litopenaeus) vannamei

Timing of organogenesis and subsequent development of genital organs were studied in female and male Pacific white shrimp Penaeus (Litopenaeus) vannamei postlarvae. This was linked to the timing of differentiation of external structures that differentiate the genders. Anatomy of the gonad appears to be unique for penaeid species. The genital organ was fully recognized from postlarve day-16 (PL16) as a bilateral lobe located in the anterior region of the midgut gland (first anterior lobes) that connects to an anterior perpendicular collector tube that extends dorsally towards the posterior region of the midgut gland and forms an inverted U-shape collector. Eight bilateral lobes in females (second to ninth) and the bilateral oviduct between the seventh and eighth lateral lobes and seven bilateral lobes in the male (second to eighth) are connected along the inverted U-shape collector tube. These lobes extend over the surface of the midgut gland beneath the pericardium. Shortly after organogenesis of the female gonad, the tenth bilateral lobe emerges from the distal region of the collector tube and continues dorsally along the intestine, and the fourth bilateral lobe did not develop and regressed until apparently absorbed. In males, the posterior bilateral vas deferens emerges from the same region. Around PL50 (0.5–0.6 g; 45–50 mm), external gender differentiation was recognized in the form of the thelycum in females and the gonopores in males. Additionally, the male androgenic gland appears at the posterior-external wall of each anterior vas deferens, surrounded by connective tissue that attaches to the anterior vas deferens and the eighth testicular lobe. Gonad differentiation occurred from PL68 (1.8–2.2 g; 70–74 mm), where it was possible to differentiate the female ovary from the male testes. Timing of sex reversal studies in penaeids is discussed.