Etude de l'effet de la complexité structurale de l'environnement sur la morphologie de l'otolithe sacculaire (sagitta) du tilapia Oreochromis niloticus

Abstract

The shape of the saccular otolith (sagitta) in teleosts is subject to well-documented genetic and environmental variations. However, ecomorphological studies have hypothesized the influence of the specifics of the environment (water depth, physico-chemical parameters, etc.), without experimentally testing any factors likely to modify sagitta shape. The objective of this master thesis is to quantify the impact of a more or less complex environment on the morphology of the sagitta of the Nile tilapia (Oreochromis niloticus). Given the dual function of the inner ear (auditory and vestibular), the auditory capabilities of the fish have been tested to determine if a variation in sagitta shape is linked to auditory function.. The experimental set-up involved rearing 160 juvenile male Nile tilapia from the same sibling for 5 months in 4 aquariums (n=40/aquarium) where only the structural complexity of the environment varied. Complexity, modeled by chains suspended in the rearing aquarium, ranged from 0 chains/m² (control condition) to 120 chains/m² (complex condition), two extremes on which we ultimately focused our analyses. At the end of the rearing period, some fish were subjected to auditory tests using the auditory evoked potential technique; the remaining fish were euthanized and dissected to recover their sagittae. Each left sagitta was photographed and measured by high-resolution digital optical microscopy (Keyence© VHX-7000) for 2D analysis and shape index calculations; these sagittae were also scanned and modeled in 3D for analysis by classical morphometric geometry - using a limited number of landmarks and semilandmarks - and high-density surface semilandmarks. Our results showed, for an equivalent body size, an increase in the size (length, width, surface area, perimeter and volume) of the sagittae of individuals from the complex condition, with no change in hearing ability. Analysis of shape indices and morphometric geometry revealed no variation in sagittae shape according to the experimental condition from which the fish were derived. While we might have suspected the influence of more complex environments (coastline, seabed, etc.) on sagittae size, this study has laid the experimental foundations, and suggests the importance of vestibular function as a constraint on otolith shape.