Differences in sagittal otoliths between two morid species: insights into the genus Physiculus (Moridae: Gadiformes)

Abstract Sagittae are widely used as tools in taxonomic and ecological studies of fishes, especially when external morphological characters are conserved or ambiguous. We analyzed morphological variation between two congeneric species of Physiculus: P. cirm, endemic to the Saint Peter and Saint Paul Archipelago, and P. kaupi, distributed along the Brazilian continental slope. Fifty adult specimens (25 per species) were examined using qualitative descriptions, morphometrics with shape indices, and 4th and 5th order wavelet contour decomposition. This work represents the first morphometric description and analysis of their sagittal otoliths. Both species exhibited sagitiform otoliths with a plane-convex profile and elliptical ostium, but the anterior region is more rounded in P. cirm and more peaked in P. kaupi, whereas the posterior region is lanceolate to rounded in P. cirm but lanceolate in P. kaupi. Statistically significant differences were found in otolith height/otolith length (OH/OL), otolith thickness/ otolith length (OT/OL), sulcus acusticus area/ otolith area (SA/OA), and otolith length/ total length (OL/TL). Classification using morphometric indices and 5th order wavelets achieved 98% accuracy, highlighting strong discriminative potential. These morphological differences likely reflect both taxonomic divergence and adaptation to deep-sea habitats, supporting otolith morphology for species identification and ecological inference in deep-sea fishes.