Otolith morphometric analysis of North Bornean Ariidae (Siluriformes): new insights and taxonomic implications

This study examines the morphometric variation of lapilli otoliths in selected modern sea catfish species (Ariidae, Siluriformes) from northern Borneo. Given the pronounced similarity in otolith shape and anatomical features among these taxa, traditional taxonomic discrimination is often challenging, particularly at the species or genus level. To assess the taxonomic utility of otolith morphology, 216 otoliths from 12 species were analyzed using high-resolution imaging, a suite of shape indices (e.g., form factor, roundness), and precise measurements of defined ventral features (e.g., linea basalis). Seventeen morphometric parameters were subjected to various statistical analyses, including Principal Component Analysis (PCA), which delineated two major groups based on otolith shape. The rounded group comprised species from the genera Hexanematichthys, Netuma (two species), and Plicofollis (two species), while the elongated group included species from Arius (three species), Batrachocephalus, Cryptarius, Kyataphisa, and Osteogeneiosus. Subsequent statistical comparisons of selected morphological parameters revealed significant differences among some taxa. Hexanematichthys sagor exhibited a distinctly circular otolith morphology, characterized by a low aspect ratio (AR) and a uniquely oriented distal edge (AP6). Within the elongated group, although the parameters varied less, their combined assessment showed promise in differentiating certain taxa, such as Cryptarius and Osteogeneiosus, particularly through measurements of the otolith central area (R1), incisura linea basalis width (AP1), and otolith thickness ratio (R2). These findings demonstrate that even subtle morphological differences in otoliths can be quantitatively resolved to enhance taxonomic differentiation among ariid catfishes.