SHARDO, J.D.*; GRANDE, T.: Morphology and Development of the Postcranial Axial Skeleton of the American Shad, Alosa sapidissima
The morphology and ontogeny of the postcranial skeleton were studied in three developmental series and a number of postmetamorphic specimens of American shad, equaling more than 180 specimens. Specimens were cleared and stained with alcian blue to identify cartilage structures and with alizarin red to examine bone development. Histological preparations were made to confirm several questionable cases of endochondrial ossification. Prehatch embryos lack postcranial skeletal elements although the finfolds and pectoral fin buds are present. By the end of the yolk-sac larval period the hypurals in the caudal fin begin to form, rapidly followed by the development of the dorsal and anal fins. As the medial fins form, the elements of the vertebral column develop. The paired pelvic fins are the last to form during the metamorphic change of larvae into juveniles. The developmental sequence was divided into a series of stages that will allow future comparisons with other closely related species. Our results also show that 1) the uroneurals in shad are endochondrial in origin like other teleosts, 2) hypural 1 is autogenous because of incomplete ossification, 3) mineralization of protocentra occurs in an anterior to posterior direction resulting in chordacentra, and 4) development of the skull and anterior vertebral column exhibit several modifications associated with the otophysic system and sound transmission.