The complex architecture of fish scale has been investigated for several decades to show an existing link between scale morphology/microstructure and systematic/evolutionary relationships of fishes. In the present study, the surface architecture structure associated with the scales of a hill stream cyprinid fish Garra shamal restricted to the Hajar Mountains of the Arabian Peninsula was examined by optical light and scanning electron microscopy; (i) to describe the whole morphology, architectural design and hidden microstructures of scales in an endemic fish of the Arabian Peninsula, G. shamal, (ii) to investigate the phenotypic plasticity of scales in six selected regions of the fish body, (iii) to show an ontogenetic pattern of scale morphology in different size classes, and (iv) to study morphometric parameters for scales, and determine the most valuable indices for differentiating the scales of different body regions and the different size groups of studied species. For this purpose, scales were removed from six body regions of three size classes. The scale of G. shamal, as an ossified platelet and hard structure covering the outer part of fish in different body parts and size groups, demonstrated numerous hidden morphological and structural characteristics, and sculptural design, for example, type, shape, and size in general term, and circuli (thickness, arrangement, and relative spacing of circuli around the focus), radii (number, shape, its origin from focus), lepidonts (the texture, shape, size, orientation, and type of its attachment), and granules (shape, size, number, and configurations) in a specific term, that can contribute efficiently in fish identification and classification at the genus and species levels. Position of focus, presence of radii in all four fields of scales (anterior, posterior, and laterals) and thus tetra-sectioned type, presence of specific tubercle arrangement on the posterior region, and some scale indices can be used as a taxonomic tool in Garra identification. Moreover, scales in G. shamal demonstrated plasticity in different fish size classes, and the origin of scales on the fish body, revealing a kind of ontogenetic variation, and the importance of key scales (scales below the dorsal fin) in the lepidological studies. The quantitative and qualitative traits described here based on the optical light microscopy and electron scanning microscopy can be implemented along with other morphological and molecular characteristics to have a better taxonomic resolution of the genus Garra. Research Highlights The optical light microscopy (LM) and scanning electron microscopy (SEM) techniques enhanced our knowledge of scale morphology in a cyprinid fish restricted to the Arabian Peninsula. The scale of G. shamal, in different body parts and size groups, demonstrated numerous hidden morphological and structural characteristics, and sculptural design that can contribute efficiently in fish identification and classification at the genus and species levels. Tetra-sectioned cycloid type, position of focus, presence of specific tubercle arrangement on the posterior region, and some scale indices can be used as a taxonomic tool for identification of the genus Garra. The scales showed plasticity in different fish size classes, and the origin of scales on the fish body, revealing a kind of ontogenetic variation.
