Linking fish ecology to otolith diversity at a broad phylogenetic scale

Ecomorphology explores the links between species morphology and its environment to better understand the adaptive value of a given phenotype. Different studies have applied this approach trying to explain intra- and interspecific variability of fish otolith shape but they did not reach a consensus about the major factors influencing this structure of the inner ear. The present study aims to uncover the major influence of teleost ecology on otolith diversification at a large taxonomic scale, spanning the fish tree of life. To do so, we (1) redefine the main axes of otolith diversity, (2) compare levels of ecological and otolith morphological diversity among orders and (3) quantify the effect size of ecological variables on otolith shape and size while accounting for phylogeny. We gathered ecological variables for 697 fish species. Collected information included species ecology (position in the water column, habitat light conditions, feeding mode, mobility and gregariousness) including traits related with acoustic communication (hearing specializations and sound production). Otolith shape was quantified with landmark-based geometric morphometric methods and phylogenetic information was retrieved from the megaphylogeny of Betancur et al. (2013). Our analysis showed that the major axes of otolith disparity involve elongation, sulcus shape, rostro-caudal thickness and curvature. Next, we found that the order-level ecological diversity is decoupled from otolith morphological diversity. Finally, we demonstrated that otolith morphology is influenced by fish occupation of the water column, habitat light conditions, and mobility. We concluded that fish otolith morphology reflects a compromise between various life history traits and cannot be summarized as a function of one aspect of a fish's ecology.