LAUSR

Abstract We investigated the life history of Ammonia “beccarii” (Linne) forma 1 in Lake Saroma, Hokkaido, northern Japan, to understand the variation in its response to a cool, brackish environment. This study was based on observations of seasonal change in its population at a fixed station (water depth ca. 1.3–2.4 m), and on its possible activation of propagules and survivorship of individuals in laboratory cultures. Small living (stained) individuals of A. “beccarii” forma 1 occurred almost throughout the year, most abundantly in February (middle of winter period of three months when the lagoon surface freezes) and June. It is likely that the small individuals of this species are not derived from the reactivation of propagules. The survivorship of young individuals in February to March was lower than in June, suggesting that “young” individuals in the cold period might be more likely to die, compared to during the warm period. In contrast, the >400 μm individuals of Lake Saroma were more likely to die at 20 °C, although they can survive well at 3 °C. Ammonia “beccarii” forma 1 of Lake Saroma appear to exhibit contrasting survivorship behavior between “young” and “adult” individuals in the cold period.