Larvae of the brackish water crab Metasesarma rubripes (Rathbun) originating from a mangrove area in Parana (southern Brazil) were reared in the laboratory from hatching to metamorphosis. Larval morphology was compared with a previous description of material from Venezuela. Morphological differences were observed and described in all larval instars, suggesting the existence of different geographical races within this widely distributed species. Moreover, M. rubripes from Brazil (unlike that from Venezuela) reveals a variability in its developmental pathways, with 4 or 5 zoeal stages preceding the megalopa. The relative frequency of these pathways within a hatch was found to differ between hatches originating from different females, suggesting that the number of developmental instars may depend on genetic or other maternal factors. Only those larvae that had passed the first three zoeal stages relatively fast (compared to sibling larvae from the same hatch) were able to develop directly from the zoea IV to the megalopa, whereas relatively delayed larvae tended to pass through a zoea V stage. Morphological comparison between zoea IV larvae developing directly to a megalopa with those passing through an additional zoeal instar shows that the former are not only faster in development but also morphologically more advanced than the latter. The zoea V of M. rubripes is described for the first time; it reveals morphological characters that are intermediate between the less advanced form of zoea IV and the megalopa. Variability in developmental pathways is interpreted a possible adaptation to extremely variable environments, where developmental flexibility may increase the chance to encounter a suitable habitat for settlement and metamorphosis.