菊池川河口域に生息するヤマトシジミの生活史および個体群動態

抄録

Corbicula japonica is one of the most dominant bivalves in macro-benthic communities in brackish lakes and estuaries throughout Japan. The population of this species in the Kikuchi River estuary, which is located in Kumamoto Prefecture, western Japan, exhibited unique mechanisms for persisting in the estuary with variable salinity of the water (0 and 30 within a day) caused by tidal activity. The planktonic larvae settled on the substrate at the upper part of the estuary, the juveniles grew up to adults, while being carried downward, and the adults occurred mainly at the lower part of the estuary. There, 2-year old adults started breeding at exceptionally small shell sizes (about 6 to 9 mm in length). However, this early reproduction seemed to give an advantage for Corbicula population to persist in the estuary. During the neap tide in autumn, reproducible adults suffer a high mortality event at the lower part of the estuary caused when high salinity water (>30) is formed on the floor of the estuary. Therefore, early reproduction enabled most adult clams to contribute to reproduction at least once in their lifetimes before they are exposed to high salinity conditions, although the number of gametes they released tended to be restricted due to their small sizes.