アユ稚魚における歯系および歯の交換

抄録

Malformations of the mouth shape, such as the“open-jaw syndrome”and“twisted lower jaw”, occur highly frequently in the ayu, Plecoglossus altivelis (Komada, 1974, 1980).It is also well known that the fish undergoes a change in the type of dentition on the jaws during the young stage (Matsui, 1938;Chapman, 1941;Iwai, 1956, 1962).In this paper, mouth shape, number and distribution of conical teeth in the juvenile stage, and period of shedding of the conical teeth and of formation of comb-like teeth on the jaws in young stage were described and reported to establish a basis for differentiating normal and ab-normal structures.The present study is based upon 800 specimens from Tanabe Bay, Wakayama Pref., and the Yahagi River, Aichi Pref., 400 specimens from Lake Biwa, 200 specimens from the Ado River, Shiga Pref., and 413 hatchery reared specimens, measuring 25-90 mm in standard length (SL).
In the juvenile stage, 30-70mm SL, the lengths of the maxillary and the dentary and the width of the vomer in fish from Tanabe Bay were larger than those from Lake Biwa.The numbers of conical teeth on the vomer, palatine and dentary in fish from Lake Biwa were significantly larger than those from Tanabe Bay (t-test, p<0.001), and the number of these teeth in hatchery reared fish was clearly reduced.But there is no difference in number of conical teeth on the elossohval and mesopterygoid between these three samples.
In 78 specimens (32.5%) of 240 hatchery reared fish, the lower jaw was twisted either to the right or left, and in 112 specimens (46.7%) the hyobranchial skeleton was ventrally projected.In these fish, the morphology of the vomer, palatine and dentary appeared normal, but the conical teeth on the bones were lacking at rates of 29.6%, 30.4% and 26.7%, respectively.
The conical teeth on the vomer, palatine and dentary were worn away or lost in 50-60 mm SL fish from the Yahagi River.On the other hand, the conical teeth were done in 66-80 mm SL fish from Lake Biwa and the Ado River.The conical teeth were shed from the bones in 50-56 mm SL hatchery reared fish.
In fish of 45-70 mm SL, the number of teeth groups on the outer surface of the jaws were significantly larger in fish from the Yahagi River than in those from Lake Biwa and the Ado River (t-test, p<0.001).But, there was no difference between the specimens from Lake Biwa and from the Ado River.In the same stage, the length of denticles in comb-like teeth on the upper jaw of fish from the Yahagi River were clearly larger than those from Lake Biwa and from the Ado River (t-test, p<0.001).There was no difference between fish from Lake Biwa and from the Ado River.