A Note on Oncorhynchus formosanum and Oncorhynchus masou

抄録

A landlocked salmonoid fish from the mountains of Formosa was described by JORDAN and OSHIMA (1919) as Salmo formosanus. Japanese workers have since considered this fish in the genus Oncorhynchus. MILLER (1950: 33) points out that since the gender of Oncorhynchus is neuter, formosanus must be written formosanum.
On the basis of scale structure, OSHIMA (1934) concluded there were no differences between O. formosanum of Formosa and the small landlocked salmon, O. masou, of southern Japan. A recent examination of specimens in the Stanford University Collection has uncovered some new evidence concerning the systematics of these fish.
A single specimen of O. formosanum was examined (S. U. 23059). This specimen showed differences from samples of Japanese landlocked salmon collected from Lake Biwa and the Daiya River, Nikko. Since the sample sizes were small, it would be unwise to place a great significance on them, but one outstanding character noticed in O. formosanum was the presence of basibranchial teeth. These teeth, often called hyoid teeth, are found in no other Oncorhynchus and only in Salmo clarkii of the genus Salmo. They were not mentioned in the original description of this fish.
If all of the landlocked salmon of Formosa possess basibranchial teeth while the Japanese salmon lack this character, they cannot be considered identical. Further work may indicate that O. formosanum should be placed with Salmo as originally proposed by JORDAN and OSHIMA (op.cit.). However, a thorough study would be neccessary to give much credence to such a decision.
Eleven specimens of O. masou were examined, 5 from the Daiya River, Nikko (S. U. 6700), labeled Oncorhynchus ishikawae, and 6 from Biwa (S. U. 6693), labeled Salmo macrostomus. The following table indicates some of the differences between O. formosanum and O. masou.