Deciphering the evolution of otolith disparity in teleost fishes

Sagittae, or saccular otoliths, are calcified structures located in the inner ear or stato-acoustic organ. The diversity of otoliths shapes observed in teleost fishes is stunning, but the evolutionary forces driving this diversity are not well understood. To study the pattern of otolith shape diversification, we used the largest dataset of teleost otoliths to date, including 697 species from 68 orders and 309 families. We used geometric morphometrics to quantify the shape of their mesial and dorsal faces and investigated evolutionary patterns with phylogenetically-informed comparative methods. Our findings indicate that otolith size and shape variation across taxa show little evidence of phylogenetic signal. The level of otolith disparity is unrelated to the age of the clades but fast-evolving clades tend to exhibit higher levels of disparity. A covariation between sulcus and otolith outline is also clearly observed. Overall, our study suggests that the tempo of sagittae evolution was not constant over time and that various factors have influenced their mode of evolution. Further research is needed to understand the factors that drive inner ear morphological evolution.