Ontogenetic development of the horn and hump of the Chinese cavefish Sinocyclocheilus furcodorsalis (Cypriniformes: Cyprinidae)

Cave adaptation has given rise to a diversity of unusual morphologies. One prominent character in Chinese cavefishes of the genus Sinocyclocheilus is the presence of a skull horn and a dorsal hump. These characters are present only in species that are troglobitic. Here we examine the osteological growth that underlies the horn and the hump of the species S. furcodorsalis using micro computed tomography. We tested the hypothesis that all structures grow isometrically, and found that the skull and the bony shelf that supports the horn grow isometrically in all dimensions. The neural spine of the first vertebrae, on the other hand, grew allometrically, getting taller and thinner with time. When comparing each structure with skull growth, the horn grew isometrically in the anterior-posterior and the medial-lateral dimensions. But the horn grew faster in height than the skull. The neural spine grew isometrically with the skull in the rostral-caudal dimension, but allometrically in the dorsal-ventral and medial-lateral dimensions. Even though the function of the horn and the hump are not known, our results suggest that heterochronic changes has led to allometric growth in these structures.