Early development of the vertebral column and the appendicular skeleton in the Inimicus japonicus

Abstract

Inimicus japonicus is one of the most commercially important marine fishes in China and Japan.The embryonic and morphological development of larvae of this species has been studied,but it is incompletely known about its osteological development.In order to obtain the knowledge of the normal onset of skeletal structures throughout development,the developmental sequences of vertebral column and the appendicular skeleton were examined in laboratory-reared larvae of Inimicus japonicus from hatching through 35 days after hatching(d.a.h)by the the clearing and staining technique of cartilage and bone.According to the observation,the vertebral column started to develop from haemal arches and neural arches at 10 d.a.h and 11 d.a.h respectively.Then,centrum,medullary spines,and pulses spines were formed at 20 d.a.h.and they were completely ossified by 35 d.a.h with trunk vertebrae flexion.The development sequence and ossification of centrum,medulllary spines,and pulses spines both started from anterior to posterior.The appendicular skeleton developed in following sequences:the pectoral fin is the first to develop,followed by caudal fins,anal fins,dorsal fins,and then ventral fins.The pectoral fin started to develop from the cleithrum and the fin plate at 3 d.a.h.Cartilages of pterygiophore of pectoral fin were formed at 11 d.a.h and some fin-supports of pectoral fins separated at 35 d.a.h.Pelvic girdle and ventral fin appeared right at 18 d.a.h,later than pectoral fin,but they are rapidly developmented.Dorsal and anal fin appeared at 20 d.a.h,and dorsal and anal fin-ray complements completed at 23 d.a.h.However,anal fin developed posterior from the middle section.The development of the caudal fin began with the appearance of the hypural at 8 d.a.h,and then caudal fin rays and fin-supports begin to develop.Caudal fin was basically formed at 15 d.a.h with two leaves a fin bone formation,which divided the caudal fin rays into two parts.The caudal fins bone formed and started preliminary ossification at 35 d.a.h.Previously-recognized critical periods for Inimicus japonicas,corresponded to transformation phases from the primitive,basic modes to stable,more functional modes in both swimming functional development and changing in living habits.The study of the development of the vertebral column and the appendicular skeleton of Inimicus japonicas will provide the basis for the functional adaptation during early development.