Discrimination of sprat species (Clupeonella spp.) in the Caspian Sea and the Sea of Marmara using otolith shape analysis

This study evaluates the discriminating power of sagittal otolith shape among three species of the genus Clupeonella endemic to the Caspian Sea (Clupeonella grimmi, Clupeonella caspia, and Clupeonella engrauliformis) and one species from the Sea of Marmara (Clupeonella Cultriventris). Identification of these species is challenging due to their morphological similarities, and the taxonomic validity of C. caspia and C. cultriventris remains ambiguous because of their resemblance. Scanning electron microscope (SEM) images were utilized to examine the morphological characteristics of otoliths. Stereomicroscope images were used to calculate six shape indices: rectangularity, roundness, ellipticity, circularity, aspect ratio, and form factor. The outline shape was analyzed using elliptic Fourier descriptors (EFDs). SEM images showed that the otoliths of all four species had an elliptical shape with a prominent and distinct rostrum. The sulcus acusticus was centrally positioned, and the anterior part of the groove (ostium) exhibited a funnel- shaped structure across all species. Distinctive morphological characteristics such as the angle between the rostrum and antirostrum (excisura ostii), antirostrum shape, and margin were noted. Although none of the otolith shape indices showed significant differences among all four species, the otoliths of C. grimmi were significantly distinct from the other three species in terms of aspect ratio and ellipticity indices. Based on the EFD analysis, the four species were strongly differentiated (85.5% accuracy), highlighting the value of sagittal otoliths in the taxonomy of the studied species. The results underscore the efficacy of otolith morphological characteristics in identifying taxonomically challenging groups—such as the genus Clupeonella—in the Caspian Sea and Sea of Marmara. This study highlights the significance of sagittal otolith shape in distinguishing closely related Clupeonella species, demonstrating its potential for resolving taxonomic uncertainties within this genus.