Three-dimensional analysis of scale morphology in bluegill sunfish, Lepomis macrochirus

Fish scales are morphologically diverse among species, within species, and on individuals. Scales of bony fishes are often categorized into three main types: cycloid scales have smooth edges; spinoid scales have spines protruding from the body of the scale; ctenoid scales have interdigitating spines protruding from the posterior margin of the scale. For this study, we used two- and three-dimensional (2D and 3D) visualization techniques to investigate scale morphology of bluegill sunfish (Lepomis macrochirus) on different regions of the body. Micro-CT scanning was used to visualize individual scales taken from different regions, and a new technique called GelSight was used to rapidly measure the 3D surface structure and elevation profiles of in situ scale patches from different regions. We used these data to compare the surface morphology of scales from different regions, using morphological measurements and surface metrology metrics to develop a set of shape variables. We performed a discriminant function analysis to show that bluegill scales differ across the body - scales are cycloid on the opercle but ctenoid on the rest of the body, and the proportion of ctenii coverage increases ventrally on the fish. Scales on the opercle and just below the anterior spinous dorsal fin were smaller in height, length, and thickness than scales elsewhere on the body. Surface roughness did not appear to differ over the body of the fish, although scales at the start of the caudal peduncle had higher skew values than other scales, indicating they have a surface that contains more peaks than valleys. Scale shape also differs along the body, with scales near the base of the tail having a more elongated shape. This study adds to our knowledge of scale structure and diversity in fishes, and the 3D measurement of scale surface structure provides the basis for future testing of functional hypotheses relating scale morphology to locomotor performance.