| We tested two sources of growth variability using a lotic species of unionid mussels Pronodularia japanensis in an agricultural drainage channel in Japan. First, the effects of physical disturbance were examined for excavation processes with short-term aerial exposures. Individuals were tagged, released, recaptured at every month, every two months, or every three months to measure growth rate. Growth rates of individuals at the end of the experiment were not affected by the intervals of excavations. Second, the effects of P. japanensis density on the growth rates of individuals were examined. Juvenile individuals (shell length < 30 mm) within experimental cages were provided with varying wet mass of adult individuals three times in different seasons. Significant positive effects of P. japanensis density on growth rates of juvenile individuals (i.e., increase in growth rate) were observed in early summer at slow-current habitat patches. Our results suggest that excavating unionid mussels at the interval as short as one month is unlikely to cause physiological stresses on individuals, and that density-dependent effects on growth rates may be important only at certain times of the entire growth period. |
