Abstract
Freshwater snails, physid (Physa acuta) and lymnaeid (Austropeplea ollula), are in an interspecific competitive relationship occupying a same niche, both of them grazing periphyton attached to the surface of aquatic macrophyte. It is known that the some species feeding a same resource can avoid the interspecific competition by food segregation resulting from differentiation of their size. In Matsumi-ike Bog at University of Tsukuba, however, it was cleared that their sizes did not have significant difference through the year. In this research, it was considered whether two snails living a same habitat have the strategy to segregate their food habitats or not by comparison between seasonal changes of generic composition of the attached and planktonic diatoms and the diatoms in snails’ gut contents. By detrended correspondence analysis of the generic compositions of diatoms in snails’ gut contents and attached and planktonic diatoms from June to December, it was showed that the individual variety of two snails’ food habitat was larger than seasonal fluctuations of the diatoms. By analysis of selective indices, it was showed that the number of diatom genera that lymnaeid grazed by selective grazing was more than that of physid did. In this research, it was revealed lymnaeid had a little different food habitat compared with physid. It is supposed that there isn’t distinct food segregation between two species, but the different food habitats between two species might be one of the causes of the coexistence in Matsumi-ike Bog.
