On the form of teeth-bearing bones and the distribution of teeth in the mouth of Oncorhynchus rhodurus fry and fingerling

Abstract

Generally, it has been long known that the tooth characters and the form of tooth-bearing bones in the mouth are valuable in the taxonomy of Salmonidae. In this paper the arrangement of teeth and the morphological changes of the tooth-bearing bones during the larval and juvenile stages in Oncorhynchus rhodurus were investigated. Two hundred and forty specimens of Oncorhynchus rhodurus, 18-66mm in standard length (SL), 7to 160 days of age, were examined in alizarin red S stained.For two months following hatching, the fish were nearly equal in body length and body weight, and, the yolk sac disappeared when the fish reached about 24mm SL, 64 days of age. Ossification of the dentary, maxillary and shoulder girdle began in about 20mm SL (24 days) and the other tooth-bearing bones in the mouth, neural and hemal spine and appendicular skeleton were calcified in 22-24mm SL (34-47 days). Ossification of vertebrae began in juvenile stage, at 25mm SL.The forms of vomer and premaxillary changed in the larval and juvenile stages (20-66mmSL). In the larval stage, 20-24mm SL, the anterior part (head) of the vomer had a notch (U or V-shaped) and ossification of the posterior shaft did not begin. But, in the juvenile stage, 24-66mm SL, the notch of the head disappeared and the posterior shaft calcified. In 22-66mm SL, the ventral surface of the head and the posterior shaft were toothed. The premaxillary was small and a slightly curved pair of bones in 20-22mm SL and it was a somewhat triangular bone in 24-66mm SL. They met in a loose median suture in the larval and juvenile stages. Each bone had 5-12 sharp conical teeth during this stage.The tooth germs on each tooth-bearing bene appeared in about 20.5mm SL, 24 days. The number of teeth on vemer, palatine, premaxillary, maxillary, dentary and glossohyal in the juvenile stage (24-66mm SL) was significantly larger than those in the larval stage (20-24mmSL)(t-test, p<0.001). In the larval and juvenile stages, tooth germs on the dentary and maxillary always appeared in the region between the two anchilosed teeth.