X-ray Microtomography Study of Cranium, Pectoral Fin and Girdle, and Weberian Apparatus Development in Juvenile Triplophysa stewarti (Nemacheilidae)

The stone loach genus Triplophysa is a major component of the Tibetan Plateau ichthyofauna, and its taxonomy and phylogenetic relationships are far from resolved. Skeletogeny patterns contain useful phylogenetic signals, but developmental data for Triplophysa are scarce. Here, we present an X-ray microtomography study of the development of the cranium, pectoral fin and girdle, and Weberian apparatus of juvenile Triplophysa stewarti (12–55 mm standard length). We reveal that the smallest juveniles possess a fontanelle and well-developed Weberian ossicles. Furthermore, the development of the cranium, pectoral fin and girdle, and bony capsule of the Weberian apparatus is illustrated, such as the suturation of preethmoid I to prevomer, the reduction of pectoral radials from four to three by fusion of the most internal two. A paired bony capsule fully enclosing the anterior swim bladder room characterizes the Weberian apparatus of nemacheilid and balitorid loaches. Our data demonstrate that the anterior wall of the bony capsule originates from a vertical plate expanded from the descending process 2, which is a homologue to the "anterior plate" in the superfamily Cobitoidea sensu lato, with the exception of Cobitidae. The lateral process 2 separates from this anterior plate. The horizontal process 4 expands anteriorly to meet the anterior wall, and when posteriorly expanded and curved downward, finally fuses with the bottom wall to close the bony capsule. Accelerated or retarded development of the aforementioned skeletal structures will result in differentiated features in Triplophysa and related nemacheilid genera.