We measured biomass and elemental composition (dry mass, W; carbon, nitrogen, hydrogen, CHN) in eggs and larvae of a mangrove crab, Sesarma rectum. The results are compared with previously published data from a closely related species with abbreviated development (S. curacaoense). Egg size of S. rectum increased during embryogenesis, while egg biomass, C content (%W), and the C:N ratio decreased. Initial biomass of eggs and larvae was in S. rectum about half of that in S. curacaoense, showing also lower percentage values of C and H and a lower C:N ratio (indicating a lower lipid content). After hatching, however, the zoeae of S. rectum showed higher growth rates than those of S. curacaoense (presumably due to partial utilisation of internal energy reserves in the latter) and a longer period of development to the megalopa (passing though three vs. only two zoeal stages); as a consequence of these differential growth patterns, S. rectum compensated for initially lower biomass. Interspecific comparison among neotropical Sesarmidae suggests an intermediate degree of dependence on planktonic food sources in S. rectum, being lower than in estuarine species with small eggs and an extended mode of development (e.g. Armases angustipes), but higher than in relatives with larger egg biomass and an abbreviated development (e.g. A. miersii, S. curacaoense, S. fossarum, Metopaulias depressus). This should allow S. rectum larvae to develop partially within the parental mangrove system, yet requiring a limited export towards coastal marine waters with planktonic food sources.