Allometric changes and shape differentiation of sagitta otoliths in sciaenid fishes

The sagitta otoliths are sound transducers that play an important role in the hearing of fishes. The shape of these structures is known to be affected by a number of genetic and environmental factors, such as depth, water temperature and substrate type. Ontogenetic allometry has also been considered an important factor in otolith shape determination. In this study, we analyzed allometric and interspecific shape variation in sagitta otoliths of five sciaenid species that occur sympatrically in northern Rio de Janeiro State (Ctenosciaena gracilicirrhus, Larimus breviceps, Paralonchurus brasiliensis, Stellifer brasiliensis and S. rastrifer), a group of fish known to have sound communication and distinctively large otoliths. The methods used are a combination of geometric and multivariate techniques, which allow for the inclusion of reference points along the otolith contour (semilandmarks) and points that are biologically correspondent among individuals and species (landmarks). The fish size and otolith size variables explained small but significant amounts of intraspecific shape variation (8-25%). The general allometric trends observed correspond to a relative increase of the ostium area and localized growth in the posterior margin, which decreases the relative width of the otoliths. These allometric shape differences might be related to environmental differences experimented by juvenile and adult individuals. The interspecific shape differences were large compared to within-species variation, even when comparing species in the same genus. The ordination of shape differences was in part related to an independent phylogenetic hypothesis but could not be related to any particular environmental variable.