Three species of Carapidae have in common a tenuis larval stage, during which they settle in the lagoon and take refuge in the same species of holothurold. From the juvenile stage, Carapus homei and C. boraborensis are commensal, whereas Encheliophis gracilis is parasitic. The aims of this study were to analyse to what extent the ontogenctic changes of the otic capsule affected the shape of the inner ear and how environmental cues, due to the above-mentioned life history and the style, could influence the structure of the sagitta. Sagittal sections revealed a three-dimensional asymmetry with a nucleus close to the proximal surface. Observations of the growth axis of the sagitta suggest it has a morphogenetic impact on the otic cavity. Each sagitta contains three main zones related to the life stages of the fish. Bands and checks were observed in the third zone in C. homei and C. boraborensis, but this pattern was less discernible in E. gracilis. These structural differences in zone 3 could be related to the commensal and parasitic life styles of these fishes. Further studies dealing with otosac features and otolith functions are suggested.