Age determination of Margaritifera laevis was carried out in the Chubu-Nougu River, at Omachi in Nagano Prefecture, Japan. There were many juvenile mussels in the brook of Chubu-Nougu River. The relationship between age and shell length in juvenile mussels was investigated. In juvenile mussels, the annual rings on the shell surface roughly indicated the true age, but the rings of the ligament did not. Therefore, it is suggested that the growth rings of the ligament should not be used in age determination of juvenile mussels.
Abnormal changes in reproduction such as a female-biased sex ratio or melanism and atrophy of the testis have been observed in Sinotaia quadrata histrica in Lake Kasumigaura, middle Japan. These changes are thought to have been induced by water pollution. In order to examine the possibility of abnormal changes of reproduction in females, we studied changes in the number of shelled embryos in the brood pouch between different seasons and regions. Females examined were collected at four sites (Ogawa, Ishida, Hasugawara, and Takayama) within the lake and at a ﬁsh farm on the lakefront as a control once a month from April 2001 to March 2002. At each collection, about 20 females were selected at random and the number of shelled embryos in the brood pouch was counted. The average number of shelled embryos during a year was 14.1 at the ﬁsh farm and 5.9 to 9.9 at the four lake sites. A clear seasonal difference between spring-summer and autumn-winter was observed at the ﬁsh farm, but only a slight difference at Ishida and Hasugawara, and no differnce at Ogawa and Takayama. In autumn-winter, the average number of shelled embryos at the ﬁsh farm was signiﬁcantly different from those at Ogawa, Ishida and Takayama. These seasonal and regional differences between the ﬁsh farm and four sites in the lake suggest the possible effect of changes in environmental conditions such as water pollution.