Shape and size diversity of sagittae in teleost fishes by using 3D landmark-based geometric morphometrics

The inner ear of teleost fishes is a complex multifunctional (hearing and balance) organ composed of three semicircular canals and three end organs having each their otoliths. In non-otophysians, the saccular otolith, i.e. the sagitta, is the biggest one and shows the largest morphological diversity. Despite many attempts to relate this disparity to hearing abilities, ecological or life history traits, there is no consensus concerning main drivers of their size and shape diversity. Thanks to the recent advances in 3D data acquisition, the use of 3D geometric morphometric methods offers new perspectives in the study of factors driving otolith shape and size, opening the way to large-scale comparative analyses with ecomorphological perspectives. Here, we aimed both (1) to apply a high-density 3D semi-landmarking technique to cover sagitta surfaces and test the sensitivity of shape quantification by using different densities of semi-landmarks (500, 1000 and 2000), and (2) to have a first glimpse of sagittae disparity in a phylogenetic framework. Therefore, 45 specimens belonging to 29 orders of non-otophysian teleosts with a particular emphasis on Carangaria and Scorpaeniformes were acquired from MorphoSource (https://www.morphosource.org/), a free online repository. Sagittae were 3D reconstructed using Amira software. We then patched points on sagittae models and the coordinates obtained were projected in a morphospace after running a General Procrustes Analysis allowing the visual exploration of their morphological disparity. Then, we increased the density of semi-landmarks on model surfaces. The increasing set of coordinates failed to improve the explained diversity and the accuracy of shape analysis. This preliminary 3D data analysis allows to validate our approach for testing further ecological hypotheses at large and small phylogenetic scales.