How a population adapts to environmental changes is a central topic in ecology, but long-term changes in the phenotype of an organism have rarely been studied in aquatic systems. In this study, we examined the morphological changes in Daphnia pulex from the initial establishment stage, using paleolimnological analysis, in Lake Fukami-ike (Nagano Prefecture, Japan). We measured morphological traits of ephippia and subfossils of D. pulex and compared the results with the population dynamics assessed in an earlier study. D. pulex established a stable population in the lake in the early 2000s. The length of the ephippia significantly increased in the mid-2000s and then decreased more recently, whereas the length of the post-abdominal claw as a proxy of body size did not show significant changes. This suggests that their adult body size was larger, and the time to maturity was delayed or the shift to the dormant phase got later in the mid-2000s, although the mean body size did not change. Subfossils of Chaoborus and the historical records suggest that planktivorous fish were less abundant and Chaoborus larvae were more abundant in the mid-2000s than in more recent periods. These results suggest that D. pulex adaptively changed phenotypic traits in response to changes in the predator community in the studied lake.
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