In this study, four species of fiddler crabs (Uca) at Inhaca Island, Mozambique, were studied: their fecundity determined and egg loss between the first and last embryonic development stage evaluated. Ovigerous female crabs were collected throughout February to March (wet season) of 2006, during day-time at low tide, in Ponta Rasa (U. annulipes and U. inversa) and Saco (U. chlorophthalmus and U. vocans) mangroves. Average brood size increased significantly with female and species size, ranging from 932 (CW = 8.51 mm) in U. annulipes, to 18,057 (CW = 23.5 mm) in U. vocans. The number of embryos decreased significantly over the incubation period for each species. Brood loss from embryonic stage I to stage IV for U. annulipes, U. inversa, U. chlorophthalmus and U. vocans was 12.3%, 14%, 43.2% and 23.6%, respectively (P < 0.05). Positive allometry was recorded for all species in the early development stage, however, due to the higher relative brood loss in larger females (U. vocans), when comparing the number of embryos in late developmental stage, a negative allometry was possible to observe. The average volume of embryos at the same stage differed among species although significant differences were found between embryonic stages for all species. Embryo volume increased 96.1%, 93.3%, 92.9% and 96.3% in, respectively, U. annulipes, U. inversa, U. chlorophthalmus and U. vocans (P < 0.05). Differences in brood loss were attributed to habitat substrate and structure, stress factors and behavioural aspects.