Does fish scale morphology allow the identification of species complex? A case study for algae scraper fishes in Iran (Teleostei: Cyprinidae)

This study investigated scale morphology in 18 Iranian algae-scraper species from the genera Capoeta and Paracapoeta to assess variation patterns and evaluate whether phylogenetic relationships correlate with scale morphological differences. Phylogenetic analysis utilized 44 COI gene sequences (651 bp) from the same specimens used for scale examination. Morphological assessment includes macro- and microscopic features from scales across five body regions and three size classes. The results revealed that the general scale type in these genera is a basal elasmoid cycloid and a sectioned type. The scales of examined species from two genera demonstrate remarkable variation in focus shape, size, and position among the five body regions and different size classes. The scales possess narrow or wide radii in three types: primary, secondary, and tertiary—the radii present in all four scale fields i.e., a tetra-sectioned type resulting in scale flexibility. The rostral margin of the scales was smooth, indented, and waved domical, dentate, and scalloped types. Additionally, the shape and size of the lepidonts varied, with some being blunt, pointed, tiny, sharp, short, and long. These scale characteristics serve as reliable diagnostic tools for species identification, classification, and phylogenetic inference within this species complex.