The influence of habitat complexity on otolith morphology and sensory capacities in Nile tilapia: a controlled experimental approach.

The inner ear of teleost fishes is known to serve both auditory and vestibular functions. Many studies have compared otoliths from different species and attempted to understand the observed differences in the light of environmental factors. However, experimental data on how otoliths could adapt are scarce. This study explores the relationship between environmental structural complexity and otolith morphology, and its effects on auditory and vestibular functions in Nile tilapia (Oreochromis niloticus). Using a controlled design, fish of the same brood were distributed between a control tank and a tank equipped with suspended chains, increasing the structural complexity alone. After a period of 5 months, we monitored auditory sensitivity and compared otolith morphology of the two groups. Auditory capacity was measured using the auditory evoked potential (AEP) technique whereas otolith morphology was assessed through classical and geometric morphometrics. Tilapias raised in complex environments developed sagittae with increased length, thickness, area and volume. However, auditory sensitivity was consistent across the two groups, suggesting that the morphological changes solely affected vestibular function. These findings support that otolith morphology can adapt to the fish's surrounding environment, here to improve balance and manoeuvrability in complex settings. This reaffirms that otolith sagittae not only support hearing but fulfil multiple roles to cope with environmental challenges. This also constitutes an indirect first demonstration of the sagitta's involvement in the vestibular function. Together with significant morphological changes, the similar auditory capabilities across groups underscore the complexity of defining functional roles within the inner ear and reaffirm the mixed-function hypothesis, challenging the association between the observed variation and inner ear-specific functions.