From fish ecology to sagitta disparity: how fish life history traits drive the morphological diversification of otoliths

Ecomorphology explore the links between species morphology and its environment to better understand the adaptive value of a given phenotype. Different studies have applied this discipline trying to explain intra- and interspecific variability of fish otolith shape without reaching a consensus on the environmental influence across teleosts. This study aims to uncover the major influence of teleost ecology on otolith diversification at a broad 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 (depth, position in the water column, mobility, gregariousness, feeding habits), body characteristics (body shape, mouth position, size class), and acoustic communication (hearing specializations and sound production). Otolith and sulcus shapes were quantified with landmark-based geometric morphometric methods. Phylogenetic information was retrieved from 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, Procrustes ANOVA revealed that otolith morphology is primarily influenced by fish size but living depth, mobility and position in the water column had minor influence. We concluded that fish otolith morphology reflects a compromise between various life history traits and cannot be summarized as a function of one particular aspect of a fish's ecology.