Population dynamics of a pollution indicator bivalve, Theora lubrica

Theora lubrica (Semelidae, Bivalvia) is a pollution indicator species in Japan. Whereas "present/absent" and "abundant/sporadic" has been discussed as environmental indices, the process in which the bivalve predominates persistently in eutrophicated waters has not been fully understood. The present study elucidated the spatio-temporal patterns of the population in Maizuru Bay, Kyoto pref., Japan, and the relationship between the patterns of the population and environmental variables were also discussed. T. lubrica abundantly occurred in the shallow innermost area of the bay where small rivers discharged terrestrial water into the sea. The population density showed a regular annual cycle. The bivalve abundantly occurred in spring, and decreased dramatically in early summer, not simultaneously with hypoxia. In autumn, the spatial occurrence was restricted to the shallow innermost area, and sporadic survivors would function as the seed population that triggers the annual outbreak of settlers in spring. The histological microscopy supported previous observation that the smallest shell length of mature females was 5 mm. Mature females were more frequent in the innermost area where more POM were supplied from the water column. The stable isotopic analysis estimated that diets for the bivalve were primary marine products. POM from the water column may be the crucial food source for the bivalve in the bay. From a numerical simulation on the budget of the POM supply and the metabolism of the bivalve, it was supposed that middle-sized individuals (4-8 mm SL) might suffer from food shortage in the central part of the bay. The survival process of planktonic larvae was calculated with a numerical simulation in which the temperature dependency of planktonic life periods was considered. Assuming that larvae were passive in the flume and hatched continuously, it was supposed that many larvae would be retained in the water column during cold days and settled massively in warming days as observed in the field, only if the larval loss through the water exchange was extremely small. Larvae would be hardly retained under the realistic water exchange in the bay, 0.07/d. Further assumptions, e.g, the active swimming of larvae, are needed for the larval accumulation in winter.