Functional relationships between benthic animals and their habitats in tidal flat [mouth of the river in Japan]

The present investigation was designed to elucidate biological production mechanism of macro benthic animals such as bivalves and crabs in tidal flat communities and intended to provide some basic information for the relationships among several sub-ecosystems geographically specialized in estuary. In order to investigate the trophic life of benthic animals and conditions of their habitat, the research work was performed in the mouth of the Natori River from 1993 to 1995. The samples were collected by the quadrat method, and surveyed environmental factors, such as g rain size, silt-clay content and the moisture of the bottom sediments, in each station. On the other hand, to make clear the feeding behavior of the animals, food items in the stomach of animals and the distribution of diatoms and detritus on the surface of tidal flat wer precisely observed. Four species of bivalves and five species of crabs inhabit dominantly in the tidal flat, forming patches parallel to the gradient of habitat conditions. In the case of Nuttalia olivacea, in accordance with growth, they vertically select different microhabitat, and they select a particular habitat in preference to others. These phenomena reveal that no two benthic species in the same general habitat can occupy for long identically the same position, that is to say, there are several different biological production processes spatially specialized in tidal flat subsystem. However, it is recognized that functional continuity as to nutritional circulation is maintained among different micro-subsystems. These nutritional materials for the production of benthi microalgae are mainly transported by tidal current and secondary produced materials are transferred by movement of benthic animals. From the results mentioned above, we can appreciate the estuary ecosystem constructed by different subsystems are basically linked through cooperation with biotic and abiotic components in the system