Growth of the Medaka (II) – Formation of Fins and Fin Appendages

In the present study, we provide information on the development of the structure and composition of fins in the medaka. The pectoral fins, which begin to form in the embryonic stage, first form their fin rays after hatching prior to the formation of other median and pelvic fins. During the growth period from larvae to juveniles, the unpaired anal, caudal and dorsal fins begin to form before the pronounced degeneration or resorption of the median fin fold in the region of the caudal peduncle. The fin rays first appear in the caudal fin fold at the posterior end of the body, and subsequently fin rays of the anal and pectoral fins arise prior to those of the dorsal fin. Paired pelvic fins begin to sprout on the skin of the bilateral sides of a median ventral fin fold, which regresses concomitant with resorption of the fin fold at the dorsal and ventral regions of the caudal peduncle. After complete resorption of those portions of the ventral fin fold in which fin rays do not arise, the pelvic fin rays form. The pectoral fins are supported by four small radials, the scapulas and the coracoids. Similarly, the pelvic fins are supported by pelvic girdles, the caudal fin by the hypural, and the anal and dorsal fins by distal and proximal pterygiophores. The appearance of these fin appendages closely follows that of fin rays in the fin fold.