Larval development

The eggs of M. rosenbergii are slightly elliptical, with a long axis of 0.6-0.7 mm, and are bright orange in colour until 2 or 3 days before hatching, when they become grey-black (Ling 1969). The larvae hatch as zoeae (Plate 1, facing p. 254), i.e. they typically utilise their thoracic appendages to swim, differing from penaeid shrimp which hatch as nauplii that are morphologically less developed and use their cephalic appendages to swim. First-stage zoeae are just under 2 mm long, from the tip of the rostrum to the tip of the telson. The zoeae are strictly planktonic, swimming upside-down within the water column and exhibiting positive phototaxy. Larvae, as well as adults, are omnivorous, mainly eating zooplankton but, in the absence of living food, they are able to feed on small particles of organic material (Ling 1969).

The larval development of M. rosenbergii is metamor-phic and typically prolonged with several instars. Variability in the number of larval stages has been observed for several decapods, including some palaemonids (Knowlton 1974; Ngoc-Ho 1976; Gore 1985). This irregular development was divided into different categories by Gore (1985). M. rosenbergii seems to exhibit 'delayed development', with 'mark-time moulting' and/or 'terminally additive staging'. In mark-time moulting the zoeal stage moults but only a small change in morphology occurs, with a slight increase in size. These zoeae, after a period of time, either die or enter a normal moulting mode and therefore complete the developmental process. Uno & Kwon (1969) reported that from zoea VI onwards a considerable variation in larval body length occurs even within the same instar, this phenomenon becoming more evident in the later stages. Other palaemonids (Palaemonetes spp. and Macrobrachium spp.)

also exhibit this type of irregular moult (Ngoc-Ho 1976). Terminally additive staging is similar to mark-time moulting; however, the supernumerary stages only occur at the end of a normal development sequence, resulting in similar individuals, differing mainly in size. According to Sandifer & Smith (1979), irregular development may confer advantages to species that live in changing environments, such as estuarine areas. This variation in the planktonic phase of M. rosenbergii may help to ensure larval dispersal, thus enhancing the chances of survival.

As a consequence of this variability, studies on M. rosen-bergii larval development also exhibit different results. Ling (1969) observed 11 moults until the larvae attained metamorphosis, but only 8 of them displayed morphologically distinct stages, while Uno & Kwon (1969) reported 11 morphologically distinct zoeal stages. From hatching to the fifth zoeal stage, each moult produces a distinct larval stage, but from the sixth stage onwards the development becomes irregular. The same occurs with M. equidens and another Macrobrachium sp. (Ngoc-Ho 1976). Both descriptions reported for M. rosenbergii coincide for the first to fifth zoeal stages, but the sixth, seventh and eighth stages in the description of Ling (1969) resemble the sixth-seventh, eighth-ninth and tenth-eleventh, respectively of Uno & Kwon

Table 3.1 Selected characteristics of M. rosenbergii larvae and postlarvae. (Modified after Uno & Kwon 1969; see also Plate 1, facing p. 254)





Antennal flagellum





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