Development, Maturation, and Function of Some Sexually Dimorphic Structures of the Malaysian Prawn, Macrobrachium Rosenbergii (De Man) (Decapoda, Palaemonidae)

The development, maturation, and function of some sexually dimorphic characteristics of Macrobrachium rosenbergii (De Man) were studied. The first evidence of sexual dimorphism is the appearance of gonopores. The smallest male had a carapace length (CL) of 5.9 mm; the smallest female had a CL of 7.6 mm. The endopods of the first pair of pleopods are sexually dimorphic, with those of the male being broad and flat, while those of the female are narrow, roundish, and possess long, fine setae. Sexual dimorphism of the endopods, as evidenced by a difference in the relative lengths of the exopod and endopod, occurs at about 12 mm CL. In the males, the appendices masculinae begin to develop at about 10 mm CL. By 13 mm CL, they attain a length of 1.7 times the length of neighboring appendices internae. This size relationship is generally maintained in future growth. Male chelipeds begin to mature at 28 mm CL, although males as large as 38 mm CL may still possess immature chelipeds. Maturation is associated with an increase in growth of mainly the merus through dactylus articles. The larger male chelipeds function primarily in reproduction by allowing a male: 1) to fend off other males intruding into its territory, and 2) to protect females, with which it has mated, from aggressive conspecifics and predators. A difference in age of maturation of the male chelipeds was noted between the present study and one done by Cowles (1914) in the Philippines. The possibility that the latter population may be a subspecies is discussed. The brood chamber and reproductive setae of females begin to develop at 20 mm CL. This may occur several molts before their first spawning. Reproductive setae are divided into ovigerous and ovipositing setae, depending upon their function. Ovigerous setae are temporary structures formed only during preparturial molts. While some ovipositing setae are also temporary, others are permanent since they are retained on nonparturial molts. A decrease in their length, however, may occur.