Sicyopterus lagocephalus is an amphidromous fish: adults live in rivers, but after hatching larvae are carried to the sea (dispersion stage). After a certain time spent at sea, post-larvae return to rivers to grow and reproduce. Sicyopterus lagocephalus post-larvae recruiting to Reunion Island rivers (Mascarene Archipelago), provide an important food source to local populations and this fishing activity has a significant socio-economic impact. A better understanding of the early life traits of this species and the characterisation of larval stages should improve the biological and physiological knowledge needed to understand the processes involved in the dispersion stage and help managers to implement conservation measures. In Reunion Island, we characterised the development of larvae from hatching to migration to the sea. The results show that larvae do not undergo any development in freshwater, and that it is the arrival at sea that triggers the morphological transformations. Our results have also revealed that development is quicker when the temperature is high and that development varies according to water depth during the downstream migration. In natural temperature conditions, the maximum survival rate in freshwater is four days. This suggests that when the downstream migration takes too long because of anthropogenic or hydraulic factors, the free embryos would die before they reach the sea.